Green’s Dictionary of Slang

kicker n.6

[it kicks the rest along]

1. (orig. US) the last, most problematical piece of information.

[US]R. Starnes Another Mug for the Bier 106: The kicker, of course, was the station wagon [...] who ever heard of having a station wagon without painting a coy name on the side of it?
[US]R. Stone Dog Soldiers (1976) 238: And the kicker is—he was never there.
[US]J. Webb Fields of Fire (1980) 358: Then there had been the kicker. ‘I can’t understand how someone could stand by and watch this happen.’.
[US]Eble Campus Sl. Oct.
[US]S. Morgan Homeboy 152: And here’s the kicker [...] this Arkie bohunk named Hutchinson actually [...] helped himself to some of that boy’s butthole.
[US](con. 1962) J. Ellroy Enchanters 76: ‘You want the kicker? Deedee’s enforcer [...] playing the role of the irate husband, is her real-life brother’.

2. a culminatory action.

[US]M. Spillane One Lonely Night 80: Maybe they were playing real cute and sent her in for the kicker.
[US]E. Torres Q&A 63: But the kicker is, while I was in the courtroom, I had this South American lady client waiting [...] She said, ‘Does this happen often in American courts?’ How do you like that shit?
[US]J. Stahl Plainclothes Naked (2002) 69: The kicker came at a dinner party Marge dragged him to.
[UK]G. Iles Turning Angel 147: But the real kicker [...] is that she didn’t just call him direct.
[UK]K. Richards Life 431: I said [...] Your sister will hate you forever. He threw in the towel. I knew that was the kicker.
[US](con. 1963) L. Berney November Road 4: The art of the payoff. Guidry understood each man’s price, the right kicker to close the deal.
[US](con. 1962) J. Ellroy Enchanters 39: He’s going There, there. Here’s the kicker.

In phrases

for kickers (also for a kicker, for the kickers)

(US) for good measure.

[US]M. Braly It’s Cold Out There 132: The trap was damaged, the man warned and now humiliated for a kicker.
[US]D. Pendleton Boston Blitz (1974) 145: He clipped on a couple of grenades, just for kickers, then got into the topcoat.
[UK]Penthouse Apr. 32: For a pledge of just $1,000 you’d get a really nice wood carving. Of course, salvation was thrown in for the kicker [HDAS].
Melissa Smith (trans.) Russian Mirror; Three Plays by Russian Women 10: [...] then for good measure flick me, and what’s more for kickers give me a lecture on morality.