In For It Meaning

(idiomatic) in trouble

Example: 1912, Stratemeyer Syndicate, Baseball Joe on the School Nine Chapter 1
  "You sure did [hit someone with the snowball]," added Teeter, stiffling a laugh. "And of all persons in the school but Professor Rodd. Oh my! Oh wow! You're in for it now! He won't do a thing to you fellows! Look at his hat! Here he comes!"