means A secret payment given to someone in return for arranging or favoring a deal, typically a slice handed back to whoever steered the money your way.

from A plain compound of "kick" + "back," American business slang of the early-to-mid 20th century. The image is literal: money flowing one direction gets a portion "kicked back" the other way, returned under the table to the person who made the payment possible. The same verb "kick back" also wandered off into the harmless sense of relaxing — same words, very different conscience.