Origin: The words Tetris (a classic famous puzzle game) and ass (the insult to somebody who is obnoxious) combined together.
1) Someone who shows no "sportsmanship" in a video game, especially when online. Specifically, those who won and is being really obnoxious. Used mainly because if the game system won't let you use the word "ass" in the game.
How to pronounce it? Say "Tet", extremely short pause, then "rass".
(A PvP battle in a MMORPG)
(Player 1 defeats P2)
P1: Hah! You're such a noob! I pwned you right there! HAHAHAHA!!!