A person who lacks street smarts/common sense. Forgetful, ditzy, or absent minded. Often used as a characteristic for a stereotypical blonde. It is important not to link airhead to stupid. It is possible to be airheaded yet have book smarts.
The carbattery is dead because she left her headlights on again, what an airhead.
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.