To Hit Armor Class Zero.
Originated from Dungeons and Dragons, where the lower the armor class, the better. An overly complicated system that almost everyone will admit sucks, especially when compared to D&D v3.x rules to hit.
Brandeth the 1st level Fighter has a THAC0 of 12.
by ShadowSiege April 14, 2004
Get the thac0 mug.