Green’s Dictionary of Slang

1. (US) Scandinavian.

1902		Bulletin (Sydney) 2 Aug. 12/3: Quits: A Salt Chestnut Illustrated. / Squarehead Sailor (throwing down a sovereign so hard that it rebounds): ‘Ya! yump, you English one poun’, yump! Das times was many I yumped for you.’.
1913		J. London Valley of the Moon (1914) 205: You remember that squarehead Champion of the United States Navy?
1948		J. Stevens Big Jim Turner 204: Every new ‘squarehead Swede’ story that came along was hung on Shot Gunderson.

2. (US) Polish.

1945		H.B. Darrach Jr. ‘Sticktown Nocturne’ in Baltimore Sun (MD) 12 Aug. A-1/1: The bars are known by the company they keep. Latins go to Spik sticks, Poles to Square (or Flathead) joints, English to Chickie dives, and so on.
1970		in P.R. Runkel Law Unto Themselves 247: We just had a little trouble down here from a squarehead dealer who hit one of my men.