Green’s Dictionary of Slang

(US black) extremely, very.

1787		R. Tyler Contrast (1790) II ii: I must needs say her father is pretty dumb rich.
1845		S. Judd Margaret (1851) I 182: He’ll have to lose his oxen if the money is’nt paid dum soon.
1914		E. Dyson Spats’ Fact’ry (1922) 34: Mean, tha’s what they are, the big boilin’ iv ’em — dumb, dirty mean!
2000		Ebonics Primer at www.dolemite.com 🌐 dum Definition: especially, a lot of. Example: Bitch, yo ass is dum fine.
2004		Urban Dict. 20 Oct. 🌐 Me: How long would it take to get from Co-op City to Fordham Road on the 12? Young Minority NYers: Dumb long, yo.
2007		D. Helene N.Y. Times 🌐 For a while they were saying, ‘That’s type funny.’ I was like: ‘What? What do you mean by that?’ It means ‘very funny.’ Or they were like, ‘That’s dumb stupid.’ I’m like, ‘That’s redundant.’.
2017		D. Winslow The Force [ebook] ‘What, you don’t like your food?’ Russo asks. ‘Are you kidding? It’s dumb good’.