Is it farther or further?
1530s, "later," from Latin posterior "after, later, behind," comparative of posterus "coming after, subsequent," from post "after" (see post-). Meaning "situated behind" is from 1630s.
"buttocks," euphemistic, 1610s, from posterior (adj.). Earlier it meant "those who come after, posterity" (1530s). Cf. Lithuanian pasturas "the last, the hindmost," from pas "at, by."
posterior pos·te·ri·or (pŏ-stēr'ē-ər, pō-)
Located behind a part or toward the rear of a structure.
Relating to the caudal end of the body in quadrupeds or the dorsal side in humans.
Near the tail or caudal end of certain embryos.