Green’s Dictionary of Slang

(US) a foolish, careless person, with a brain full of air.

[1863		in T.P. Lowry Stories the Soldiers Wouldn’t Tell (1994) ) 157: Bubblehead says the war will be over soon].
1949		Collier’s Mag. 123 34/1: He's portrayed as a bubblehead Plain Dad, or possibly, Father Can Be Beautiful.
1950		J. Morrison Port of Call 56: It ain’t much, but it would be better than stoppin’ wiv old Bubble-’ead.
1952		Lait & Mortimer USA Confidential 186: The Washington bubblehead and boodle crowd.
1968		G. Cuomo Among Thieves 181: Just Bubblebrain up to his usual tricks.
1975		L. Rosten Dear ‘Herm’ 9: The bubblehead resites [sic] ‘The number is 6-4-1-8848’.
1982		G.V. Higgins Patriot Game (1985) 159: He goes out and marries this bubblehead.
1991		M.B. ‘Chopper’ Read Chopper From The Inside 112: As for ‘Bubble Brain’, if he has ever won a fight then I certainly haven’t heard about it.
1992		J. Mowry Way Past Cool 25: She wondered who rode shotgun with the little street prince, some ebony airhead, or maybe a blond bubble-brain.
2006		C. Hiaasen Nature Girl 86: Sammy Tigertail was depressed to think that this bubblehead would soon be a schoolteacher.
2013		T. Dorsey Riptide Ultra-Glide 158: ‘He won’t be able to resist showing off for the bubbleheads’.