What is the X in X-mas?
"the seat bone," 1640s, from Latin, from Greek iskhion "hip joint," in plural, "the hips," probably from iskhi "loin," of unknown origin.
ischium is·chi·um (ĭs'kē-əm)
n. pl. is·chi·a (-kē-ə)
The lowest of the three major bones that constitute each half of the pelvis, distinct at birth but later becoming fused with the ilium and pubis. Also called ischial bone.