OP – Over powered. Like the Golden Gun in Golden Eye.
Quickscope – Quickly switching between a scoped view and non-scoped view in between firing a round (or rounds, depending on the weapon.)
Noscope – See previous, but without the scoping.
360 noscope – see previous, but while spinning 360 degrees before shooting.
Hacker – someone who appears to be using illegal software modifications to improve their performance in the game. (most of the time people do not actually hack the game but are simply such superior players inferior players get salty.)
Ragequit – Finding an uber annoyance in losing and quitting game violently. (usually online)
Speedrun – Completing a game as quickly as possible without regard for the story. Sometimes involves glitching.
Glitching – Abusing the game’s technical design flaws to your advantage.