Much like a twink. An uber-twink is an MMORPG (mostly WoW) veteran who's character has a bunch of top-shelf equipment from their older, maxed-out characters, but who also dominates the BG's (Battle Grounds) with great standing and fervor.
OMFG!... I just cleared out 12 orcs solo in the BG's!!! My toon's an uber-twink!!!