Special thanks to all the TOR.nsibs who made suggestions and gave definitions - I couldn't have done it without them. Further corrections and suggestions for additions welcome; just e-mail me by clicking the CONTACT button to the right!
I have divided these into three categories: general Tolkien-related, fanfic/writing, and message board. There is a certain amount of overlap, though, so if you're looking for a particular word it's wise to check them all. I've capitalized the way that I do it, but others may capitalize differently... e.g. FOTR instead of FotR.
Tolkien-related:
JRRT: J.R.R. Tolkien, the man himself
ME or M-e: Middle-earth
LotR: The Lord of the Rings
FotR: The Fellowship of the Rings
TTT or TT: The Two Towers
RotK: The Return of the King
App. or Appendices: the appendices at the end of RotK, often specified as App. B (or whatever)
Sil or Silm: The Silmarillion (published posthumously)
UT: Unfinished Tales (published posthumously)
HoMe: The History of Middle-earth (12 posthumous volumes, edited and with extensive notes by Christopher Tolkien)
Letters: The Letters of J.R.R. Tolkien (edited by Humphrey Carpenter)
CT: Christopher Tolkien, son of JRRT
CSL: Clive Staples Lewis, author of the Narnia books and friend of JRRT
Fanfic/writing:
If you have corrections or suggestions to make, please e-mail me! The following terms are mostly general, although a couple have specific Tolkien examples appended. Thanks to all who have made suggestions or contributed definitions.
For a site with a far more extensive list, try The Fanfiction Glossary. Fannish Definitions also has useful information.
AKA: also known as
AN or A/N: author's note
angst: fanfic genre dealing primarily with themes of anxiety, guilt, remorse – that is to say, angst!
AU: alternate universe, i.e. non-canon
BD or B&D: bondage and domination
BDSM: bondage/domination/sadism/masochism
beta: a draft, see WIP. As a verb, it means to read such a draft and offer constructive criticism for improvement
beta reader: someone who betas another author's work
canon: the world according to the original author; there is (arguably) a LOT of Tolkien canon, definitely including the Hobbit and LotR, (probably) the Sil and UT, and (possibly) also the HoMe volumes, despite internal contradictions; in Tolkien fandon there is also a distinction between book canon and film canon
CD: character death; can also refer generally to a fanfic in which a canon character dies
CO: see XO
crackfic: a story that seems to have been written when the author was on crack; something so awful that you can't believe it was written, but you can't stop reading anyhow
drabble: a piece of exactly 100 words, plus up to 15 words for a title
fanfic: fan fiction; pretty much what it says, stories/poems/etc. written by fans of a particular author
fanon: information developed in a fanfic rather than in the original work, but which has come to be widely accepted and used by other fans in their writing; calling Haldir the "Marchwarden of Lórien" is fanon, for instance, as neither Tolkien (book) nor even Peter Jackson (film) names him so
filk: a song rewritten to fit into a fandom; originally occurred with folk songs at fan conventions
F/F: female/female; see slash
gen: "general" story in which romance or erotica plays no significant part; can be of any genre except romance
HC or H/C: hurt-comfort, subgenre of angst in which the express purpose of the story is for one character to get hurt so that another can comfort them and bring them back to health, either emotionally or physically
het: heterosexual romance or erotica
HNC: hurt no comfort, AKA “get ’em,” another subgenre of angst, in which the sole purpose of the story was to beat up on one of the characters, usually in loving detail, both emotionally and physically, with little or no comfort
kill: see CD
Mary-Sue: fanfic in which the main character is the author thinly disguised; also refers to that character. If the character/author is male, sometimes called Gary-Stu
M/M: male/male; see slash
OOC: out of character
OC: original character, i.e. non-canon, invented by fanfic author for purposes of a particular story
OFC or OMC: original female character, original male character
OTP: One True Pairing, that is, whatever pairing a given reader or author prefers, sometimes to the point of refusing to read/write about any others
parody: an imitation of the characteristic themes or styles of a particular author or work, usually exaggerated and applied to an inappropriate subject for the purpose of humor or ridicule
POV: point of view
PWP: any of several phrases, all with similar meanings: Porn Without Plot, Positively Without Plot, Plot What Plot, etc.; can be either het or slash
ratings: typically fanfic archives rate stories with a system based on that of films (most often the American film rating system), thus G, PG, PG-13, R, NC-17. R and NC-17 are normally considered "adult" ratings, the others "general."
RPS: real person slash, e.g. a story with a romance between Dominic Monaghan and Billy Boyd, as opposed to one between Merry and Pippin
R/R or R&R: read and review
RW: real world
self-insertion: a form of Mary-Sue in which the author uses her/his own real name (or handle) for the OC
slash: story with non-canon, homosexual romantic or erotic relationships between characters - these may or may not be explicit*
SM or S&M: sadism and masochism
songfic: story based on, and usually incorporating quotations from, a contemporary pop song
Sue: see Mary-Sue
XO: crossover, combining two different fan universes in the same story (e.g LotR and Harry Potter)
WIP: work in progress
*Note on slash: The precise definition of slash is up for some debate. Some would define it as any M/M or F/F pairing, regardless of whether such a pairing is canonical. Some only define it as a non-canonical such pairing - this is my preferred definition, which emphasizes the discursive, boundary-breaking nature of the work. Also, the order in which the two names are placed in the abbreviation can be meaningful; if one partner is dominant sexually, that person's name is placed first (hence Aragorn/Boromir means something slightly different from Boromir/Aragorn). However, not all authors pay attention to this distinction.
Extra note on Tolkien slash. Typical slash is A/L, A/B, or F/S (Aragorn/Legolas, Aragorn/Boromir, and Frodo/Sam respectively) but there are an awful lot of other miscellaneous pairings out there. There are even a few Mary-Sue (Gary-Stu?) slashes, although most Mary-Sues are resolutely heterosexual, presumably reflecting their creators.
Messageboard:
Note: Some of these are specific to TOR.n, but some are commonly used. I have not included abbreviations to various TOR.nsibs' nicks; this list would become unmanageable if I did! They're usually pretty self-evident, anyway. Nor have I included terms that are usually found in chatrooms rather than message boards; the number of acronyms, specialized terms, and so forth used in chat seems to multiply every time one turns around, and I can't possibly keep up even if I wanted to. One of the great things about TOR.n is that use of correct English is strongly encouraged so that those who know it as a second (third, fourth) language will be able to participate more easily, but there are a few abbreviations that do get used often, which I list here.
Admins: forum administrators, who have the authority to delete messages deemed inappropriate and even ban trolls if necessary
Arena: the Arena board at TOR.n
BB: Billy Boyd (Pippin)
b/c: because
BTW: by the way
DM, Dom: Dominic Monaghan (Merry)
Elwood, EW: Elijah Wood (Frodo)
Figwit: Elf sitting next to Aragorn at the Council of Elrond (in the film); name stands for "Frodo Is Great... Who Is That?; TOR.nsibs inDUHvidual and Arwenelf, who began what has, astonishingly, become an international phenomenon, created a Figwit web site, now maintained by inDUH. Figwit reappears as Arwen's escort in RotK, and is played by Bret McKenzie
fwoar: expression of lust
IEC: invisible extra credit, given on quizzes for humorous answers
IIRC: if I recall correctly
IM, IMcK, Sir Ian: Sir Ian McKellan (Gandalf)
IMO, IMHO, IMNSHO: in my (not so humble) opinion
j/k, JK: just kidding
JRD: John Rhys-Davis (Gimli)
LOL: laughing out loud, or lots of laughs
Maiar: see Admins ;-)
Main: the Main TOR.n message board
MB or Movie: the Movie discussion board on TOR.n
MIA: missing in action
OB, Orli: Orlando Bloom (Legolas)
OT: off topic (on some boards it can mean on topic)
OTOH: on the other hand
PHF, PEF, PMF, etc.: pervy hobbit fancier, pervy elf fancier, pervy man fancier... see VSD
PJ: Peter Jackson (LotR film director, duh!)
RL: real life
ROFL, ROTFL, ROTFLMAO: rolling on the floor laughing (my ass off)
RR: the Reading Room discussion board on TOR.n
SA: Sean Astin (Samwise)
SB: Sean Bean (Boromir)
snert: verb of unknown origin, meaning approximately somewhere between a snicker and a snort
SW: Star Wars
TOR.n: TheOneRing.net
TOR.nadoes, TOR.nsibs: those who hang around at TOR.n, on the message boards or in the chat rooms
TPTB: the powers that be, i.e. the creators of TOR.n
UUT: utterly unsupportable theory
TMI: too much information
Valar: see TPTB
VM: Viggo Mortensen (Aragorn)
VSD: the Very Secret Diaries of assorted LotR characters (slash!), originated by Cassandra Claire; various TOR.nsibs have been known to write them as well
W/O: without
WWGD: what would Gandalf do?
YMMV: your mileage may vary