Green’s Dictionary of Slang

squarehead adj.2

[squarehead n.1 (2)]

(US) stupid.

[US]E. O’Neill Anna Christie Act I: johnny: (picks up the letter [...] reads very slowly) Christopher Christopherson. the postman: Square-head name.
[US]J. Conroy World to Win 200: Hey, you squarehead Polack.
[UK]W. Hall Long and the Short and the Tall Act II: The army’s full of square-head yobs who keep their brains between their legs.
[US](con. 1970s) G. Pelecanos King Suckerman (1998) 36: Squarehead cops and jealous boyfriends and barroom bad-asses.
[Aus]M.B. ‘Chopper’ Read Chopper 4 214: Who can beat who in a fist fight is a squarehead topic.