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.
[1920s+] (US campus) a remedial course in elementary English composition.
[1910s+] (US) an elementary, obvious error or mistake; thus pull a bonehead play, to make an elementary error.
see pull a boner under boner n.3