What is "ischial callosity"

Ischial \Is"chi*al\ ([i^]s"k[i^]*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.

n. A thickened piece of skin found on the buttocks of animals, especially the baboon.