How do you pronounce THAC0?

Heh.
I can tell you that Gary Gygax explained it to a table of GenCon gamers, in my hearing, as "THACK-oh." And that's the only way I ever heard old guard designers at TSR pronounce it. I, too, have heard other pronunciations since.

— Ed Greenwood (@TheEdVerse) July 14, 2021