Green’s Dictionary of Slang

1. (US) a thrill.

1941–2		Gach In Army Now 179: What a kicker we get out of receiving a package! [HDAS].
1962		P. Crump Burn, Killer, Burn! 119: That’s the kicker, I mean, not knowing.
1984		Algeo Stud Buds and Dorks 5: Something that is very good...awesome...the real kicker [HDAS].

2. (US black) a bonus.

1978

R.D. Pharr Giveadamn Brown (1997) 185: ‘The real kicker is that thirty keys of dynamite “H” go with the deal’.

3. (US) a chaser, usu. pertaining to drugs rather than drinks.

1990		M. Berent Steel Tiger 252: Marijuana cigarettes with a hash kicker [HDAS].
2001		David Bryant All-Dayer [Screenplay on Internet] angus: We need a kicker. How about a round of tequila. I need to get back to being newted.
2001		YellowRoomNet Archive 28 Apr. 🌐 Best Acid ? Any of it. All of it. But, Lemon Wafers with a hash kicker was a hoot.