Green’s Dictionary of Slang

(US) a consequence or hidden twist.

1941		in R. Riskin Meet John Doe (1989) 136: [film script] And here’s the kicker for you – we were in the same outfit .
1971		N. Cassady First Third 77: The kicker is that since Mae was hardly 13 [...] the Denver Post thought their nuptial ceremony newsworthy.
1978		A. Maupin Tales of the City (1984) 92: Here it comes [...] She’s always got a kicker.
1986		L. Heinemann Paco’s Story (1987) 131: The kicker was that even the fucking rubble was worthless.
2000		F. Kellerman Stalker (2001) 489: Maybe he thought he had beat the system, because the crime was still unsolved an entire year later. But there was a kicker.
2013		J. Stahl Bad Sex on Speed 19: Under the soft drink, in small print, the kicker: Is it marketing if my new wife does it?