1530s, "later in time," from Latin posterior "after, later, behind," comparative of posterus "coming after, subsequent," from post "after" (see post-). Meaning "situated behind, later in position than another or others" is from 1630s. Related: Posterial.
"buttocks, the hinder parts of the body of a human or animal," euphemistic, 1610s, from posterior (adj.). Earlier it meant "those who come after, posterity" (1530s). Compare Lithuanian pasturas "the last, the hindmost," from pas "at, by." Middle English had partes posterialle "the buttocks" (early 15c.), from Latin posterioras with a change of suffix.