Green’s Dictionary of Slang

1. (orig. US) a fool, a stupid person.

1928		Daily Express 4 Dec. 10/3: A ‘dumb’ is a stupid person, and if he’s dumb enough he’ll probably drive you ‘cuckoo’ or crazy.
1967–8		Baker et al. CUSS 110: Dumb A person who always does the wrong thing. A person without much social or academic ability.
1989		H. Rawson Dict. of Invective (1991) 130: dumb/dummy. Stupid; a brainless person.

2. stupidity.

1993

T. Jones Pugilist at Rest 148: Quit giving me dumb. I’m sick of that fucking act. Use your head.