Green’s Dictionary of Slang

cabbage-headed adj.

also cabbage-head
[cabbage-head n. (1)]

foolish, stupid.

[US]D. Corcoran Pickings from N.O. Picayune (1847) 10: A low, chubby cabbage-headed Dutchman.
[UK](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.
[US]N. Heard House of Slammers 89: Dum-Dum and Cabbagehead Dick.