Know how to use "fewer" and "less"? Find out.
"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.