Green’s Dictionary of Slang

1. (US) an immigrant from northern Europe, esp. a Swede; thus adj. roundheaded [? physiognomy].

1895		DN I 393: Roundhead, a Swede.
1899		Anaconda Standard (MT) 23 Apr. 2/5: Carson said to Dahlman, ‘You round-headed Swede s— b— ’.
1903		S. Clapin New Dict. Americanisms 341: Roundhead. In the North-West, frequently said of a Swede.
1930		‘Dean Stiff’ Milk and Honey Route 38: Swedes are ‘roundheads’ or ‘salve eaters.’.
1938		G.G. Korson Minstrels of the Mine Patch 317: Roundhead: Nickname for Slavic immigrants.
1949		Monteleone Criminal Sl. (rev. edn).
1966–9		in DARE.
1988		(con. 1930s) C.E. Lincoln The Avenue, Clayton City (1996) 56: Goddamn German krauts [...] Roundhead bastards trying to take our democracy.

2. (usu. teen) a circumcised penis; thus the boy or man who has one [the antonym of cavalier n.; 17C cites, with no ref. to circumcision, pun on contemporary roundhead / cavalier divisions].

[1647		H. Neville Parliament of Ladies 3: They […] voted, that no round head should dare come into any of their quarters].
[1658		Mennis & Smith ‘The Bursse of Reformation’ Wit Restor’d (1817) 139: Heers childrens bawbles and mens too, / To play with for delight / Heer’s roundheads when turn’d every way / At length will stand upright].
1959		personal recollection.
1984		Partridge DSUE (8th edn) 991/2: C.20.
1996		K. Lette Mad Cows 95: Penises, like snowflakes, are each of them different. And Maddy liked them all. [...] The round-heads. The hooded eyes. The meat and two veg packed lunch variety.
2001		OnLine Dict. of Playground Sl. 🌐 roundhead n. circumcised penis.
2019		P. Baker Fabulosa 290/1: Cavaliers and Roundheads uncircumcised and circumcised penises.

3. (drugs) any drug contained in a capsule with curved ends.

1980

E. Folb Runnin’ Down Some Lines 168: Diluted dealer compounds were referred to as [...] round heads, or flat heads.