Green’s Dictionary of Slang

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

1971		Daily Times-News (Burlington, NC) 17 Aug. 9A/7: We are not all air-heads [OED].
1977		Eble Campus Sl. Apr. 1: airhead – a stupid person, usually applied to females.
1986		Totally True Diaries of an Eighties Roller Queen 🌐 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.
1999		Guardian Media 28 June 8: You do realise she is a total airhead.
2013		A. Sim ‘Through the Perilous Night’ in ThugLit Feb. [ebook] ‘He’s obviously an airhead’.
2016		C. Eble (ed.) UNC-CH Campus Sl. Fall .
2021		P Howard Braywatch 3: ‘Leo Varadkar.’ ‘An airhead – minding the class in the absence of a grown-up!’.

2. (US campus) an intelligent person.

1997

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

3. (drugs) a marijuana user.

2001

ONDCP Street Terms 1: Airhead — Marijuana user.