Green’s Dictionary of Slang

squarehead adj.1

[squarehead n.2 ]

1. (US) Scandinavian.

[Aus]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.’.
[US]J. London Valley of the Moon (1914) 205: You remember that squarehead Champion of the United States Navy?
[US]J. Stevens Big Jim Turner 204: Every new ‘squarehead Swede’ story that came along was hung on Shot Gunderson.

2. (US) Polish.

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.
[US] 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.