Green’s Dictionary of Slang

1. a fool, an ignoramus.

1922		E.C. Litsey Bluegrass Cavalier 205: Your head should have been filled with brains instead of black mud, Sir Thickwit.
1948		G.W. Gabriel I Thee Wed 171: ‘I’d be a thickwit if I hadn’t.’ ‘Sensed and — sympathized with them.’.
1963		P. Lyon Success Story 157: Some barely literate thickwit on the staff of the syndicate.
1986		M.D. Hart Jasmine on the Wind 330: Of a certain that azure-eyed, stubborn, scowling thickwit whom she adored would surely return from Egypt.
1990		T. Winton Human Torpedo 9: He doesn’t have any, thickwit.
2003		G. Keyes Briar king 484: That’s good, coming from a thickwit like you.

2. attrib. use of sense 1.

2003

R.T. Smith Hollow Log lounge 38: No thickwit bouncer can treat me like this.