The Collaborative International Dictionary of English v.0.48:

Butment \But"ment\, n. [Abbreviation of Abutment.]
   1. (Arch.) A buttress of an arch; the supporter, or that part
      which joins it to the upright pier.
      [1913 Webster]

   2. (Masonry) The mass of stone or solid work at the end of a
      bridge, by which the extreme arches are sustained, or by
      which the end of a bridge without arches is supported.
      [1913 Webster]

   Butment cheek (Carp.), the part of a mortised timber
      surrounding the mortise, and against which the shoulders
      of the tenon bear. --Knight.
      [1913 Webster] Butt