Green’s Dictionary of Slang

(US) anything or anyone outstanding or remarkable, often used ironically.

1908		N.-Y. American 9 Aug. in Fleming Unforgettable Season (1981) 161: Pittsburgh may be a one-man team, but that man is a ‘dilly.’.
1941		N. Davis ‘Don’t Give Your Right Name’ in Goulart (1967) 36: The juries they hatch up in this state – and do we have some dillies!
1951		M. Spillane One Lonely Night 29: They were a fine pair, those two, a brace of dillies.
1959		W. Burroughs Naked Lunch (1968) 61: The Mau Mau [...] are said to be cooking up a real dilly of a VD for the white folks.
1968		K. Brasselle Cannibals 83: This unexpected decision was a dilly.
1978		S. King Stand (1990) 468: Mankind’s final traffic jam had been a dilly.
1994		Eble Campus Sl. Apr. 3: dilly – someone unusual.