Green’s Dictionary of Slang

squarehead n.2

[SE square + head; ? the severe ‘Prussian’ haircuts]

1. [20C+] a German or one of German origins .

2. [20C+] (Aus./US) a Scandinavian.

3. [1930s] (US) a Pole.

4. [1970s] (Can.) an Anglophone Canadian.