Although this topic hadn't been replied to for a while, I know some key twinking terms that aren't said.
BiS = Best in Slot. The best equips for your level. Weapons, shields, armor, etc.
SiS = Second in Slot. The second best, usually twinks will go for these if they're easy to get, and then work to getting BiS.
GF/Grandfathered - An item passed down that is no longer in game, given by a Game Master or somebody who stocked up on them!
Looms/BoA - Heirlooms/Bind on Account
Agil - Agility
Int/Intell - Intellect
Sp - Spellpower
sta/stam - Stamina
AGM - Arena Grand Master, an item you get after turning in 12 AMs, this trinket gives 12 stam and can be aquired 3, 6, 9, 12 AM/PM, more than once.
AM - Arena Master, used once you get 12 from competing in Gurubashi Arena to make an Arena Grand Master.
ST/RT - Server Time / Realm Time. Server and realm time can be checked under your minimap.
Guru - Gurubashi Arena
Rum - Rumsey Rum Black Label, or sometimes Haalani Whiskey, Haalani Whiskey
GD - Good duel
GG - Good Game
GS - Goldshire, the Alliance central of dueling.
Xroads - Crossroads, found in the Barrens. Center of dueling on the Horde and World PvP is common there depending on your server.
SW - Stormwind, the Alliance Capital, home of the XP Eliminator
Org - Orgrimmar, the Horde Capital, home of the XP Eliminator
XP - Experience Points
XP Eliminator - Turns off Experience, a must for twinking. Costs 10 gold.
WSG - Warsong Gulch
AB - Arathi Basin
AV - Alterac Valley