Green’s Dictionary of Slang

1. (orig. US) a strange and foolish person.

1981		Frank Zappa ‘Conehead’ 🎵 Conehead...she ain’t really dumb / She’s just a Conehead...’tater chip crumbs / All over her face.
1983		S. King Christine 156: Is there any more juice, or did you two coneheads drink it all?
2006		C. Carr Our Town 272: He stood there in his white robe and hood, ignoring the little kid who kept coming up to say ‘Hi, Conehead’.

2. (N.Z. prison) a skinhead.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 46/1: conehead n. a skinhead.