Green’s Dictionary of Slang

onionhead n.

1. (US) a stupid person; thus onion-headed, stupid [-head sfx (1)].

[US]Daily Dispatch (Richmond, VA) 25 Jan. 1/5: First it was Jowler, the crusty old bachelor [...] and then the onion-headed ‘Readjuster,’ Bangle, ever-susceptible to feminine influence.
[US](con. 1918) L. Nason Sergeant Eadie 46: You and that big onion head been layin’ up somewhere!
Day & Lay SAC [film] Take over, onionhead [HDAS].

2. (US) in senses of a smooth, peeled onion.

(a) a bald-headed person [SE head].

[US]T.J. Farr ‘The Language of the Tennessee Mountain Regions’ in AS XIV:2 91: onion headed. Bald headed.
[US](con. 1930s–50s) D. Wells Night People 118: Onion head. Onion-shaped head.
[US]Greenville News (CA) 19 May 5/4: How many times have you had to answer to ‘Baldy’ [...] ‘Onion-Head,’ ‘Chrome-Dome’ or ‘Slick?’.

(b) a close, shaved haircut.

Amun-Ra ‘Letters From Junebug Johnson’ on African Amer. Poetry and Discussion Forum Archive 🌐 You would have cracked up when the cop put his hand on that big onion head and pushed him into the backseat. It looked like a baby trying to palm a basketball. I almost laughed out loud watching that big head squeeze through the rear door of the police car.