Green’s Dictionary of Slang

1. German.

1888		W.B. Churchward Blackbirding In The South Pacific 13: ‘Those square-headed Dutchmen,’ as we call all Germans.
1919		letter in M. Baldwin Canteening Overseas (1920) 190: They do acrocbatics [sic] [...] by the Rhine, causing the square-headed population to stare with astonishment.
1921		(con. WWI) E. Lynch Somme Mud 39: Come on, you square-headed bastards!
1937		J. Archibald ‘Crash on Delivery’ in Flying Aces Nov. 🌐 Sneak over, will ya, ya square-headed Boche!
1938		X. Herbert Capricornia (1939) 113: Old Karl Fliegeltaub, benign, bespectacled, square-headed old outcast.
1944		L. Glassop We Were the Rats 197: What the hell’s wrong with ya, yer square-headed bastard?
1959		J. Gosling Ghost Squad 178: The abulance [...] took me to the German Hospital. I was there [...] for ten days before that square-headed doctor tumbled that my shakes were permanent.
2003		Steve Earle Just an Amer. Boy [album] I used to get the shit beat out of me by great big squareheaded cowboys called Otto.
2010		IOL News (Western Cape) 26 Aug. 🌐 The modern British stereotype of the Germans presents them as autocratic, unimaginative and ‘squareheaded’ [...] which refers both to a rigid way of thinking and a short militarty haircut.

2. Scandinavian, esp. Swedish.

1912		Van Loan ‘Little Sunset’ in Ten-Thousand-Dollar Arm 95: Two days passed, and the Swede did not appear. [...] ‘Why, the big, square-headed dog!’ said Pete.
1919		Van Loan ‘Nine Assists & Two Errors’ in Score by Innings (2004) 407: Show me the team of squareheaded Swedes [...] that can drive thim from the box!
1981		G. Johnston Fish Factory 156: Come to your squareheaded old Uncle Jippy.
2001		H. Kizer Like Rain on Kupreanof 167: You’re a smart kid, but you are hard headed, like a square headed Norski.