What is the X in X-mas?
also kick-back, c.1900 in various mechanical senses, from kick (v.) + back (adv.). By 1926 in a slang sense of "be forced to return pelf, pay back to victims," which was extended to illegal partial give-backs of government-set wages that were extorted from workers by employers. Hence sense of "illegal or improper payment" (1932).
Money given to someone illegally or unethically: Buying another poor devil's job for $50 or a kick-back from his pay/ All the cops were on the pad, getting kickbacks from the hookers (1934+)