getting absolutely destroyed in some kind of competition or event, usually in a videogame like Halo. pwn is derived from "own" but the o is replaced with a p, whiched is supposed to represent a tongue sticking out, therefore forming the word "pwn". I pwn noobs all day. but back to the uber part, uber is a synonym for "super", but it usually has a negative connotation.

When you eat dirt for the first time and leave your family to build a shack in the woods

Something that happens when someone is being beaten by double their score or more. Originated in Modern Warfare 2: Free-For-All. The person that did this is now allowed to brag about it and make fun of the person and the other guy can't say anything because everyone knows he sucks at the game.