I've played one ADnD game. The rest were all run in 3rd or higher. The only reason I remembered THAC0 was because I remember being really confused by the whole thing. In my first reply, I'd started to explain what I thought it was... thought better of it, and wiki'd... Turns out that was a good idea, since my original thought was all wrong. I remembered the term though! Sounds like thwack-o, which works out well for a mnemonic device.