Green’s Dictionary of Slang

(US) a fool.

1968		H. Sackler Great White Hope III v: Clinch him, dummox.
1972		D. Smith Report from Engine Company 82 9: I don’t mean it that way, dummo, I mean about the investigation.
2001		‘DUMMO FAQ’ Dummo.com 🌐 Q What’s a dummo? A If you thought someone was dumb, you might call them a dummo. We don’t think that you are dumb, though. Q Well, then is it you guys who are dummos? A No, we’re pretty smart for Internet millionaires.