Green’s Dictionary of Slang

cupola n.

[SE cupola, a rounded vault or dome forming the roof of any building or part of a building]

(US) the head.

[US]A.H. Lewis ‘Mulberry Mary’ in Sandburrs 10: Mebby he gets funny in his cupolo, bein’ up so high.
[US]Alaska Citizen 28 July 8/4: You never will land a Johnny-boy that’s got enough gray matter in his cupola to want a real, bang-up flossy lady for his kiddo.
‘Our Boarding House’ 27 Dec. [synd. comic strip] Smoking (bad cigars) [...] finally smogged his cupola [W&F].