Tasha, Suijin, Anons', Spyrle and Vargna have provided entries. If you'd like to provide some make a comment. Will update this post and put credit where its deserved.
++: Invulernable Save
d6: Roll 1 six sided dice
2d6: Roll 2 six sided dice
40K6: Warhammer 40,000 6th edition. Read same for other editions. 40K5 = fifth edition.
AA: Anti-Aircraft. Weapons that have Skyfire rule.
AB: Attack Bike, Space Marines.
AC (AssC): Assault Cannon, Space Marines.
Alpha Strike: Tremendous amount shooting meant to cripple, if not greatly hinder, the opponent in the first round of shooting.
AoBR: Assault on Black Reach. 5th ed starter box.
AP: Armour Penetration. If weapons AP is less than Sv target cannot roll Save.
AtA: Air to Air
AtG: Air to Ground
ATSKNF: and They Shall Know No Fear, Space Marine.
BAO: Bay Area Open. A tournament ran by Frontline Gaming in San Francisco.
BatRep: Battle Report. Details of a game.
BFS: Battle for Salvation. Grand Tournament held in NY.
BGB: Basic Game Book.
BLOS terrain: Blocking Line of Sight terrain
BP: Bolt Pistol
BotBG: Blessing of the Blood God, Khorne Gift
BRB: Basic Rulebook
Bubblewrap: Unit or units used to protect other units. q.v. 12.5.9 BatRep Necron vs Chaos Daemons
BYB: Big Yellow Book. 6th ed corebook.
CC: Close Combat, also Chaos Cultist
CCB: Catacomb Command Barge, Necrons.
CCW: Close Combat Weapon
CD: Chaos Daemons
Chimera ML/HF: Chimera with Multi-laser & Heavy Flamer, Imperial Guard.
Chronotek, Crytpek with Chronometron, Necrons.
CML: Cyclone Missile Launcher, Space Marines.
CoD: Cities of Death. Games Workshop buildings.
CSM: Chaos Space Marines
DAoT: Dark Age of Technology. An age of the Imperium.
DAngels (DA): Dark Angels
Dark Harlistar: Harlequin heavy Dark Eldar list.
DAVU, Dire Avengers as Vehicle Upgrade. Where for 60 points a min sized squad of troops inside a Falcon made it scoring (5th ed). Credit: Spyrle
DBJ: Don't be a Jerk! Tag we put on back of CONvergence badges. Be cool, don't piss off fellow players thus maintaining potential gamers for future games.
DCA: Death Cult Assassins, Inquisitor Henchmen.
DCCW: Dreadnought Close Combat Weapon.
DE: Dark Eldar
DK: Dreadknight, Grey Knights.
DKOK: Death Korps of Krieg, Imperial Guard.
DH: DaemonHunters. 3rd ed Codex convering Grey Knights and Inquisitors.
DoA: Descent of Angels. Ability of Blood Angels re-roll failed deep strike and scatter d6 instead of 2d6
DoC: Daemons of Chaos
DoM: Doom of Malan'tai, Tyranids
DoW = Dawn of War, another of the 3 mission types in the game. Credit: Suijin. 5th ed.
DP: Daemon Prince.
DS: Deep Strike (40k6 p36)
DT: Designated Transport (40k6 p78)
DtW: Deny the Witch. (40k6 p68)
DV: Dark Vengence. 6th ed starter box.
DW: Draigowing, a Grey Knight army list.
EADL: Eldar Aegis Defense Line. (Keep up the good work Spyrle)
EML: Eldar Missile Launcher (40k6 p414)
FA: Fast Attack, Force Organisation Slot.
FAAC: Fashion at all Costs. Building an army based on fashion not winning. (This is mostly what I do, though I'm trying to balance the sides)
FAQ: Frequently Asked Questions.
(F)LGS: (Favourite[Friendly]) Local Gaming Store. The place where your wallet goes to die. Friendly credited to Tasha.
FF: Flickerfield, Dark Eldar.
FoR: Fortress of Redemption.
Flyrant: Flying Hive Tyrant.
FMC = Flying Monstrous Creature. Credit: Anonymous 12.7.31 10:35AM
FNP: Feel No Pain. Special Rule (40k6 p35). Ability to avoid being wounded.
FOC: Force Organization Chart
GEQ: Guard Equivalent: Lots of 5+ or worse saves (low stats)
GH: Grey Hunter, Space Wolves
GK: Grey Knights
GKSS: Grey Knights Strike Squad
GKT: Grey Knight Terminator.
GS: Green Stuff
GT (Grand Tournament): Regional event that usually consists of 2-3 days of 40k battles.
HF: Heavy Flamer.
HnA: Hammer and Anvil. One of three deployments in 6th.
HP = Hull Points. credit: Vargna
HtP: How to Paint a digital download.
IC: Independent Character
KFF: Kustom Forcefield, Orks.
KoS: Keeper of Secrets, Chaos Daemons.
KP: Kill Points. Same as Victory Points. q.v.VP
LC (LasC): Lascannon
LG: Liquifier Gun, Dark Eldar.
LoF: Lord of Fire, Necrons.
LOS: Line of Sight
LoS: Look Out, Sir. (40k6 p16). Not to be confused with LOS. Note lowercase "o".
LRBT: Leman Russ Battle Tank, Imperial Guard.
LS: Land Speeder, Space Marines.
Mathhammer: Statistics people use to determine the result of dice rolled one unit vs another. q.v. http://www.heresy-online.net/combatcalculator/shooting.php (warning ESET anti-virus says something about the site having a Trojan and disables that tab, I use Firefox)
MC: Monstrous Creatures, Force Organisation Chart.
MEQ: Marine Equivalent: 3+ save (high stats)
Meta: Play style/army build. Example: 40k Community in Jacksonville mostly use range armies. 40k Community in Dallas mostly use Hand-to-Hand armies.
MoK: Mark of Khorne
MoN: Mark of Nurgle
MoS: Mark of Slaanesh
MoT: Mark of Tzeentch
MSS: Mind Shackle Scarabs, Necrons.
MSU: Multiple Small Units. Elites, Troops, Fast Attack in squads less than 10. Designed to spam the field and provide target saturation. Target saturation provides a tremendous amount of units for your opponent to shoot at thus limiting the effectiveness of shooting at squads.
NDK: Nemesis Dread Knight, Grey Knights.
PA: Power Armour
Paladinstar: A Grey Knight army using Paladin's which are notoriously hard to kill.
PBS: Psyker Battle Squad, Imperial Guard.
PC: Psycannon i.e. Paladin 1 PC Banner. Terminator Grey Knight with Psycannon and Banner.
PGL: Phantom Grenade Launchers, Dark Eldar.
PS: Power Sword.
Psyback: Razorback with psyammunition, Grey Knights.
Psyrifle Dread (Psyflemen): Grey Knights Dreadnought Autcannons with psyammunition.
PT: Personal Teleporter, Grey Knights.
Pulseteks: Crypteks with Solar Pulse, Necrons.
RAW: Rules as Written. Example: Mawlock is a large blast weapon therefore cannot hit flyers.
RCL: Random Charge Length
RoW: Runes of Warding, Eldar.
RJB: Reaver Jetbike, Dark Eldar.
RT: Rogue Trader, 1st ed 40k.
RTT: Rogue Trader Tournament.
RW: RavenWing, Dark Angels.
SAFH: Shooty Army From Hell. Typically referring to Imperial Guard because of the number of shots.
SB: Storm Bolter i.e. Paladin 1 SB Banner. Terminator Grey Knight with Storm Bolter and Banner.
SC: Splinter Cannon, Dark Eldar.
SiTW: Shadow in the Warp, Tyranid.
SnP: Slow and Purposeful. Special Rule (40k6 p42)
Soladin: Solo Paladin. Grey Knights
SoB = Sisters of Battle (AKA red-headed step children of the Imperium) Credit: Suijin.
SP: Shock Prow, Dark Eldar.
Sport: Short for Sportsmanship. How one presents themselves during a game.
Sv: Save, have to roll equal to or greater than number on d6 to avoid being wounded.
SW: Space Wolves
S weave: Sempiternal Weave, Necrons
TA: Terminator Armour
TA Squad: Tactical Assault Squad, Space Marines.
TAC: Take all Comers, a list built to fight any army list.
Tabled: Every model has been removed from the table.
Tremorteks: Crypteks with Tremorstaves (Thanks IDICbeer)
TLAC: Twin-linked Assault Cannon
TLHvyB (TLHB) : Twin-linked Heavy Bolter
TLLC: Twin-linked Lascannon
TLPC: Twin-linked Plasma Cannon
That Guy: A gamer who is a rules-lawyer-*ssh#l@ and/or that is out to ruin other peoples fun.
tic tacs: Little white breath tablets you use to win.
TEQ: Terminator Equivalent: 2+ save (high stats)
ToS: Tournament of Skulls.
TS: Temporal Snares, Necrons.
TH/SS: Thunder Hammer/Storm Shield, Space Marine.
USR: Universal Special Rules. i.e. Feel No Pain, Scout, etc. 5th ed.
VB: Venom Blade, Dark Eldar
Vindi: Vindicator, tank.
VoD: Veil of Darkness, Necrons.
VotLW: Veteran of the Long War, Chaos Space Marines.
VP: Victory Points. Example: Slay the Warlord Secondary Objective yields 1 VP, unless Purge the Alien then 2 VPs.
WAAC: Win at all Costs. A method to abuse game mechanics to win the game.
WIP: Work in Progress, stages a model goes through when being put together and painted.
WLT: Warlord Traits.
WoC: Warriors of Chaos
WS: Warscythe, Necrons.
WW AA: Whirl Wind Anti-Aircraft, Space Marine.
WWP: Webway Portal, Dark Eldar.