Green’s Dictionary of Slang

bonehead n.1

[SE bone + -head sfx (1); the image is of the hardness of bone; Moore, Lexicon of Cadet Language (1993), suggests an alternative link to bone n.1 (1a) on pattern of dickhead n.]

1. [20C+] (orig. US) a fool, a dullard, an idiot; thus as a term of address.

2. in attrib. use of sense 1.

3. [1920s–60s] a stubborn person.

4. [1920s+] a stupid error.

5. [2000s] (S.Afr.) an Afrikaaner; also as adj.

In compounds

bonehead English (n.)

[1920s+] (US campus) a remedial course in elementary English composition.

bonehead play (n.) [play n. (2); note locus classicus the ‘bonehead play’ committed by Giants 19-year-old Fred Merkle rookie substitute in Giants vs. Cubs game on 23 Sept. 1908; the term was coined by the NY press which headlined ‘Merkle’s Bonehead Play’]

[1910s+] (US) an elementary, obvious error or mistake; thus pull a bonehead play, to make an elementary error.

In phrases