Green’s Dictionary of Slang

airhead n.

[SE air/air n. + -head sfx (1)/-head sfx (4)]

1. [1970s+] (US, orig. teen) an idiot, a fool, someone who has nothing but air, and no brains, in their head.

2. [1990s+] (US campus) an intelligent person.

3. [2000s] (drugs) a marijuana user.