posterior (n.) Look up posterior at Dictionary.com
"buttocks," 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."
posterior (adj.) Look up posterior at Dictionary.com
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.