This is a fascinating project and an excellent diversion for me, so here's my contribution. I hope you find it useful and/or enlightening.
Screenwatching/Screenloooking: When playing against another person who shares the same television/computer screen as you do, the act of observing the other player's movement and minimap to gain additional (often considered unfair) advantages.
Ragequit: To exit a game altogether in a fit of rage, esp. as a reaction to perceived unfairness or repeated defeat.
Scrub (uncommon secondary definition): A player who deliberately refuses to get better at a given game through available means such as guides, tutorials, or videos. Distinctive from "noob," which generally refers to a new or otherwise unskilled player. See David Sirlin's Playing to Win [http://www.sirlin.net/ptw] for more information.
Frak: mild expletive; commonly substituted for "fuck" as a more acceptable interjection. See also "gorram." (from Battlestar: Galactica)
Turtle/Turtling (clarified): a (usually defensive) strategy adopted by a single player or team to take advantage of a geographic location (corner, choke point, etc).
Gorram: Substitute for "god damn." From Joss Whedon's Firefly/Serenity.
Killsteal (clarified): To claim illegitimate credit for the defeat of an opponent, usually by dealing the killing blow on a target already weakened by another party.
Hax: 1) Informal term for (usually illegal) third-party modifications to game software, intended to create advantages that are otherwise technically impossible (infinite health/ammo, increased speed to nigh-teleportation, etc). 2) An interjection voicing awe/jealousy of the luck of another player.
Ninja: 1) To steal items intended for another. Usually seen in cooperative online games/activities. 2) One who acquires items in this way. 3) One skilled in stealth-based gaming.
MMORPG: Massively Multiplayer Online Roleplaying Game. Colloquially, Mostly Men Online Roleplaying as Girls. Commonly abbreviated to MMO.
Multiboxing: To run simultaneous copies of a game from the same computer, allowing one to execute multiple actions as several different avatars at the same time (or at least within margins of time not feasible for those using multiple computers/players). Occasionally illegal; however, some companies (such as Blizzard) do not object due to the fact that each separate avatar requires an active account to maintain.
Spam (clarified): To use a single or narrow selection of abilities/items, esp. while pressing the buttons for these abilities faster than the game can process them.
Dragon Kill Points: 1) A form of currency in MMORPGs, used to determine priority and bid on loot from a completed encounter. Commonly referred to as DKP. 2) A loot prioritization system involving this currency. (From Everquest)
Nerf (clarified): The weakening of a game mechanic by the developers in order to achieve internal balance.
Buff: The strengthening of a game mechanic by the developers in order to achieve internal balance.
Soon: A colloquial term describing the time between the present and the end of the world. The assumed, albeit tongue-in-cheek, timeframe by which game developers release highly-anticipated content. (From Wowwiki, copied from the official forums. [http://www.wowwiki.com/Soon]) See also: Valve time. [http://developer.valvesoftware.com/wiki/Valve_Time]
Gank: To assault (with intent to kill) another player via an ambush or other tactical advantage.
PvE: Player versus Environment.
PvP: 1) Player versus Player. 2) Player versus Pillar, jokingly derived from the strategy by which one player hides behind an obstacle (usually a pillar) in an attempt to force one's opponent into a more advantageous position.
LoS: 1) Line of Sight. 2) To move out of this, forcing an attacking opponent to reposition/change strategy.
Carebear: 1) A player who avoids PvP gameplay. 2) (In online games) A server dedicated to PvE, such that PvP functionality is disabled in the majority of the game. (Derogatory)
Rape (clarified): To defeat another player/enemy within the context of a game, in a generally highly one-sided skirmish. Highly distinct from the act of forced sex upon another living being. Usually announced as an expression of celebration ("I raped him!") or acknowledgment of defeat ("We got raped").(Editor's note: Skill in execution has inherently nothing to do with this otherwise informal term)
Own: See Rape.
Pwn (clarified): See Own. Origin unknown; suggested to be a spelling error pronounced phonetically.
Woot: An exclamation of jubilation or excitement. Origins unknown and widely speculated on.
Guild: See clan (in original post).
Vent: Abbreviation for Ventrilo, a Voice Over Internet Protocol (VoIP).
TS: Abbreviation for Teamspeak, a Voice Over Internet Protocol (VoIP).
HUD: Heads-Up Display, the visible part of a gaming interface that relays information such as health, ammunition, equipment, etc. to the player.
FPS (expanded): 1) Abbreviation of the First-Person Shooter game genre. 2) Frames per second.
Mod: 1) A third-party application, potentially but not inherently illegal, intended to give a cosmetic or functional change to the game. 2) To apply such an application to one's game. 3) A moderator of an online community.
Banhammer: A fictional object used by a moderator to exile a player from an online community.
Additions pending.