What is the X in X-mas?
"buttocks muscle," 1680s, from Modern Latin glutaeus, from Greek gloutos "the rump," in plural, "the buttocks."
gluteus glu·te·us (glōō'tē-əs, glōō-tē'-)
n. pl. glu·te·i (glōō'tē-ī', glōō-tē'ī')
Any of the three large muscles of each buttock, especially the gluteus maximus, that extend, abduct, and rotate the thigh.