Green’s Dictionary of Slang

1. (US und.) a committment [despite recorded chrono., this would seem to be a fig. use of money-based senses].

1915

Pittsburg Dly Post (PA) 20 Apr. 6/2: He was impelled to reform by the ‘spiels’ of the superintendant of the refuge [...] he has made his ‘kick-in’, wants to make a man of himself — and that [...] is ‘no funny stall’.

2. a financial donation, a tip.

1922

Broadway Brevities Aug. 20/1: Five orchestras are said to have retired to summer homes at Larchmont on the 5 a.m. kick-ins of the Wolf.

3. (US) a commission on a payment made by the payee to the customer, usu. a genteel euph. for a bribe.

1938		W. Winchell On Broadway 23 Aug. [synd. col.] Roosevelt support for Maryland’s Lewis is having a tremendous kickback (or is it kick in?) from Big Biz.
1952		W.R. Burnett Little Men, Big World 29: Has to be a kick-in, of course.

4. a bribe.

1963

in T.I. Rubin Sweet Daddy 58: This syndicate stuff and the cop kickins [...] with the houses and all.

5. a contribution.

1996

S. Maloney Brush-Off (1998) 107: Our kick-in was $300,000.