Green’s Dictionary of Slang

1. (US) a general pej., suggesting stupidity; also attrib.

1906		L.A. Times 4 Nov. 41/4: Boxers may come and boxers may go, but the slanthead handler etc. Some men have no more idea of howto second a boxer than a rabbit.
1913		Chicago Trib. 7 Sept. 28/1: [cartoon caption] ‘Why you slanthead, I can see it now right now under here!’.
1917		Dly Gate City (Keokuk, IA) 25 Jan. 2/3: To hell with peace talks [...] while those slantheads [i.e. Germans] across the line are enslaving French [...] women and children.
1920		C. Sandburg ‘Cahoots’ in Smoke and Steel 45: Go fifty-fifty. / If they nail you call in a mouthpiece. / Fix it, you gazump, you slant-head, fix it.
1927		(con. 1918) J.W. Thomason Red Pants 71: Plain fool! [...] I had to send Sergeant Hamilton out there to secure that flank, an’ the slant-head got himself killed.
1939		Capital Times (Madison, WI) 22 Aug. 18/1: A resolution rich in vitriol and invective [...] was adopted [...] even if the legislative slantheads didn’t understand much of the polysyllabic language.
1946		H.A. Smith Rhubarb 267: That big slanthead sitting over yonder.
1964		Missoulian (MT) 30 Jan. 19/3: Filbert Slanthead, that jovial [...] lad with the vacant stare.

2. (US) pej. term for an Asian, also attrib.

1965

J. Hersey White Lotus 192: Listen Mei-Mei, this is a slanthead world [Ibid.] 450: Why did I behave like a slanthead? Why couldn't I just do whatever I wanted?