kick in

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kick in (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.

   Upon hearing residents in the burning house, the passerby kicked in the front door and yelled to those inside.

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.
   • 2013, Richard Linklater, Julie Delpy, Ethan Hawke, Before Midnight (motion picture), Julie Delpy (actor):

     People expect women [when they give birth] to have this instinct that kicks in.

   • 2005, Sean Dooley, The Big Twitch, Sydney: Allen and Unwin, page 289:

     Once the wet kicks in up north, you can be stranded for months waiting for swollen rivers to subside to a crossable depth[.]

   You have to push the switch hard to get the heater to kick in.

   I took my medication an hour ago, and it hasn't kicked in yet.

4. (transitive and intransitive, idiomatic) To contribute, especially to a collection of money.

   For the year-end party, we're asking each employee to kick in twenty dollars.

   This is a worthy charity, so everyone should kick in.

• kick in the pants