Green’s Dictionary of Slang

squarehead adj.²

(US) stupid.

1920		E. O’Neill Anna Christie Act I: johnny: (picks up the letter [...] reads very slowly) Christopher Christopherson. the postman: Square-head name.
1935		J. Conroy World to Win 200: Hey, you squarehead Polack.
1959		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.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 36: Squarehead cops and jealous boyfriends and barroom bad-asses.
2005		M.B. ‘Chopper’ Read Chopper 4 214: Who can beat who in a fist fight is a squarehead topic.