Green’s Dictionary of Slang

thick head n.

a hangover.

1917		A.S.G. Lee letter in No Parachute (1968) 25 May 26: Then comes the chorus: For a batman woke me from my bed, / I had a thick head and a very sore head.
1946		D. Stivens Courtship of Uncle Henry 122: In the morning he could not remember very much and he had a thick head.
1952		M.F. Caulfield Black City 43: You shouldn’t drink whisky [...] That sort of stuff gives you a thick head.