Ischial \Is"chi*al\, a. (Anat.) Of or pertaining to the ischium or hip; ischiac; ischiadic; ischiatic.

Ischial callosity (Zo["o]l.), one of the patches of thickened hairless, and often bright-colored skin, on the buttocks of many apes, as the drill.

Source: Webster's Revised Unabridged Dictionary (1913)