The Collaborative International Dictionary of English v.0.48:

Ischial \Is"chi*al\ ([i^]s"k[i^]*al), a. (Anat.)
   Of or pertaining to the ischium or hip; ischiac; ischiadic;
   ischiatic.
   [1913 Webster]

   Ischial callosity (Zool.), one of the patches of thickened,
      hairless, and often bright-colored skin, on the buttocks
      of many apes, as the drill.
      [1913 Webster]