And here, in no particular order, is the forum dictionary.
OP: In context means different things.
OverPowered: Plasma rifle is OP!
Opening post: Yes, that
Opening poster: Who posted opening post; OP is an idiot.
-Yoink-: Get the fudge out
-Yoink!- or -blam!-: Replacement for swearword
WTF: What the fudge
FTS: Fudge this shot!
FPS: First person shooter
Troll: A person deliberately spreading misinformation or annoying people.
RPG: Rocket Propelled Grenade=Rocket Launcher
DMR: Designated Marksman Rifle
ROFL: Rolling on floor laughing
LOL: Laughing out loud
:P, :D, 
etc. Smiley
jfxngsnhyo: Person is frustrated
Close/lock: End posting in thread
343/343i/343 Industries: Makers of halo. 
eg; From Moa’s post…: Abbreviating usernames using key words
H2, H3, HR etc.: Referencing Halo game using acronym
Bump: Move a thread to the top; get it noticed
Sticky: A permanently first page thread
K/D: Kills to deaths. Normally used by trolls or tryhards as their last line of defence to try to prove that their opinion and their fun matters more because they are “pro”.
Tryhard: Person who thinks that other people’s fun is invalid because they are better.
W/L: Wins to Losses. Often used similarly to K/D.
RROD: Red Ring of Death: Occurs to damaged/failing Xboxes.
OMG: Oh My Gosh/God: Normally used sarcastically.
EE: Easter egg. In game secret.
Skull: In game gameplay modifier.
FF: FireFight.
Halo 3: Recon: ODST’s original name.
Hope this helped. 
have a nice day.