Green’s Dictionary of Slang

airhead n.

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

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

[US]Daily Times-News (Burlington, NC) 17 Aug. 9A/7: We are not all air-heads [OED].
[US]Eble Campus Sl. Apr. 1: airhead – a stupid person, usually applied to females.
[Can]Totally True Diaries of an Eighties Roller Queen [Internet] 23 Mar. Today I was talking to Rob and Raul phoned him and asked him why I dropped him. He’s such an air head. What a dork.
[UK]Guardian Media 28 June 8: You do realise she is a total airhead.
C. Eble (ed.) UNC-CH Campus Sl. Fall .

2. (US campus) an intelligent person.

[US]Da Bomb Summer Supplement 1: Airhead [...] Smart guy.

3. (drugs) a marijuana user.

[US]ONDCP Street Terms 1: Airhead — Marijuana user.