to cave in

from The Collaborative International Dictionary of English v.0.48

Cave \Cave\, v. i.
   1. To dwell in a cave. [Obs.] --Shak.
      [1913 Webster]

   2. [See To cave in, below.] To fall in or down; as, the sand
      bank caved. Hence (Slang), to retreat from a position; to
      give way; to yield in a disputed matter.
      [1913 Webster]

   {To cave in}. [Flem. inkalven.]
      (a) To fall in and leave a hollow, as earth on the side of
          a well or pit.
      (b) To submit; to yield. [Slang] --H. Kingsley.
          [1913 Webster]