Tasha, Suijin, Anon, 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
AA: Anti-Aircraft. Weapons that have Skyfire rule.
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.
ATSKNF: and They Shall Know No Fear
BAO: Bay Area Open. A tournament ran by Frontline Gaming in San Francisco.
BatRep: Battle Report. Details of a game.
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 Necron vs Chaos Daemons.
BYB: Big Yellow Book. 6th ed corebook.
CC: Close Combat
CCW: Close Combat Weapon
CD: Chaos Daemons
Chimera ML/HF: Multi-laser Heavy Flamer
CSM: Chaos Space Marines
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. 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 maintain potential gamers for future games.
DCA: Death Cult Assassins
DE: Dark Eldar
DK: Dreadknight, Grey Knights
DH: Daemon Hunters. 3rd ed Codex consisting of Sisters of Battle and Inquisitors.
DoA: Descent of Angels. Ability of Blood Angels re-roll failed deep strike and scatter d6 instead of 2d6
DoM: Doom of Malan'tai. Tyranids
DoW = Dawn of War, another of the 3 mission types in the game. Credit: Suijin.
DS: Deep Strike
DtW: Deny the Witch. (40k 6th p68)
DW: Draigowing, a Grey Knight army list
EML: Eldar Missile Launcher (40k 6th p414)
(F)LGS: (Favourite[Friendly]) Local Gaming Store. The place where your wallet goes to die. Friendly credited to Tasha.
FAQ: Frequently Asked Questions.
Flyrant: Flying Hive Tyrant.
FMC = Flying Monstrous Creature. Credit: Anonymous 12.7.31 10:35AM
FNP: Feel No Pain. Special Rule (40k 6th p35). Ability to avoid being wounded.
FOC: Force Organization Chart
GEQ: Guard Equivalent: Lots of 5+ or worse saves (low stats)
GK: Grey Knights
GKSS: Grey Knights Strike Squad
GS: Green Stuff
GT (Grand Tournament): Regional event that usually consists of 2-3 days of 40k battles.
HP = Hull Points. credit: Vargna
HtP: How to Paint digital download.
LOS: Line of Sight
LoS: Look Out, Sir. (40k 6th p16). Not to be confused with LOS. Note lowercase "o".
LRBT: Leman Russ Battle Tank.
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)
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
MSU: Multiple Small Units. Elites, Troops, Fast Attack in squads less than 10. Designed to spam the field and provide target saturation. Target saturation is providing a tremendous amount of units for your opponent to shoot at.
NDK: Nemesis Dread Knight
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.
Psyback: Razorback with psyammunition.
Psyrifle Dread (Psyflemen): Grey Knights Dreadnought with psyammunition on Autocannons.
RAW: Rules as Written. Exampe: Mawlock is a large blast weapon therefore cannot hit flyers.
RCL: Random Charge Length
RT: Rogue Trader 1st ed 40k
RTT: Rogue Trader Tournament. Smaller scale tournament.
SB: Storm Bolter i.e. Paladin 1 SB Banner. Terminator Grey Knight with Storm Bolter and Banner.
SiTW: Shadow in the Warp. Tyranid.
SnP: Slow and Purposeful. Special Rule (40k 6th p42)
SoB = Sisters of Battle (AKA red-headed step children of the Imperium) Credit: Suijin.
Sport: Short for Sportsmanship. The way one presents themselves during a game.
Sv: Save, have to roll equal to or greater than number to avoid being wounded.
SW: Space Wolves
Tabled: Every model has been removed from the table.
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.
TH/SS: Thunder Hammer/Storm Shield
USR: Universal Special Rules. i.e. Feel No Pain, Scout, etc.
VoD: Veil of Darkness. Piece of wargear that allows Necrons to deep strike instead of normal move.
VP: Victory Points.
WAAC: Win at all Costs. A method to abuse the game mechanics to win the game.
WIP: Work in Progress, stages a model goes through when being put together and painted.
WoC: Warriors of Chaos.
WWP: Webway Portal