Green’s Dictionary of Slang

cabbage-headed adj.

foolish, stupid.

1845		D. Corcoran Pickings from N.O. Picayune (1847) 10: A low, chubby cabbage-headed Dutchman.
1914		G. Herriman Dingbat Family 22 Apr. [synd. cartoon strip] Y’ big cabbage faced gazooni.
1935		(ref. to 1870) Dundee Courier 6 Apr. 6/3: Have we lost the art of slang? [...] It did not pay to be stupid in those days [...] you had ‘apartments to let’ or [you were] chuckle-headed, buffle-headed, cabbage-headed [and] chowder-headed.
1983		N. Heard House of Slammers 89: Dum-Dum and Cabbagehead Dick.