Definition of that's it
1 —used to say that something is finished or completed Okay, that's it. You can go now.
2 —used to say that something is all that is needed or wanted I came here just to visit the museum and that's it.
3 —used to say that something is correct "I can't remember his name." "I think it was Brian Johnson." "Yes, that's it."
4 —used in an angry or annoyed way to say that one will not accept any more of something That's it! No more horseplay in the house.!