Green’s Dictionary of Slang

lighthead n.

[SE light + -head sfx (1); the supposedly minuscule weight of the fool’s brain]

a fool, a simpleton.

[[UK]‘An Amateur’ Real Life in London II 230: A Charley about three parts sprung, and who appeared to have more light in his head than he could shew from his lantern].
E. Glyn Man & Maid 39: My niece Madelaine — a lighthead — dragged me to the Ritz to lunch last week.
Collier’s Mag. 127 49/2: What a lighthead I was then, dancing and dashing around.
[Ire]T. Murphy Whistle in the Dark Act II: Blood, and fighting, and light-heads, and daft fathers, and mad brothers!

In derivatives

lightheaded (adj.) (also light-brained)

stupid, foolish, mad.

[UK]Marlowe Edward II line 2109: The proud corruptors of the light-brainde king Haue done their homage to the loftie gallowes.
[UK]R. Burton ‘Democritus to Reader’ Anatomy of Melancholy (1893) I 75: Old men account juniors all fools [...] Italians French-men, accounting them light-headed fellows.
Hexham & Manly Copious English and Netherdutch Dictionary n.p.: Kiecken-hoost, Giddy -headed fellow or Light-brained man.
[UK]Proceedings at Sessions (City of London) Apr. 19/1: He had attempted to drown himself, and was taken out of the Thames. Jane Saunders and Mary Ray depos’d, That about two Months since, he had told them, he had been Light-headed, and that they thought he look’d Melancholy.
[UK]Monthly Rev. Aug. 143: By what enchantment [...] is it that so light-brained, frivolous a people, have been able to extend over the universe, the ruinous and tyrannical empire of its modes?
[UK]G.A. Stevens Adventures of a Speculist II 121: The Light-headed ones are those foot-stools of folly called Poets.
G. Baretti Dizionario delle lingue italiana, ed inglese n.p.: Cervellino [...] hair-brained, brainless, light-brained.
British Rev. Aug. 495: The reputation of Great Britain abroad, when degraded by the floating swarms of her light-brained citizens.
T. Gaspey Self-Condemned 35: So look to it, hot-brained, hare-brained, light-brained, moth-brained stormer!
T.C. Grattan Beaten Paths I 91: A gay, light-brained young fellow.
[UK]M. Lemon Leyton Hall 43: Maud is wrong [...] to hold these morning levees of light-brained idlers when I am absent.
J.M. Saxby Preston Tower 1 47: Now for the owls and the bats, and may they impart some of their own sagacity to this light-brained noddle of mine.
[UK]Sporting Times 15 Mar. 2/2: Being assured that our sleeper would go right through, it was perfectly natural to have to change at Paris, for such is the way in this land of light-headed gibbering gonophs.
C. London Total Package 11: Sidney Blakely only wanted to escape the coy, perfumed, primping, light-brained mass of calendar models at the Amoteh Resort.