Definition of "kick in"

kick in

verb
third-person singular simple present kicks in, present participle kicking in, simple past and past participle kicked in
1. (transitive) To kick or strike so as to cause the object struck to collapse or fall inwards.
2. (transitive, slang) To kick or strike (a person); to beat up
3. (intransitive, idiomatic) To start, connect, or take effect, especially in a sudden way.
  Quotations
  People expect women [when they give birth] to have this instinct that kicks in.
  2013, Richard Linklater, Julie Delpy, Ethan Hawke, Before Midnight (motion picture), Julie Delpy (actor)
  Once the wet kicks in up north, you can be stranded for months waiting for swollen rivers to subside to a crossable depth[.]
  2005, Sean Dooley, The Big Twitch, Sydney: Allen and Unwin, page 289
4. (transitive and intransitive, idiomatic) To contribute, especially to a collection of money.