Green’s Dictionary of Slang

(US) I/you/he/she/we/they thought; also as n.

1887		Lantern (N.O.) 15 Oct. 3: Who’d a thunk it!
1922		Joyce Ulysses 483: Stop twirling your thumbs and have a good old thunk.
1932		J.T. Farrell ‘A Practical Joke’ in Short Stories (1937) 173: How many wells make an ocean. Who’d a thunked it?
1936		A.E. Duckett ‘Truckin ’round Brooklyn’ in N.Y. Age 18 July 7/1: Getting together [...] Thelma Brown and Kenny McGee...Who’da thunk it?