Young Girl With A Nice Ass Buttocks

• hurdies - British, origin unknown, also applied to the whole rump
• haunches
• moon was a common shape-metaphor for the butt in English since 1756, and the verb to moon meant 'to expose to (moon)light' since 1601, long before they were combined in US student slang in the verb (al expression) mooning "to flash the buttocks" in 1968.
• prat (British English, origin unknown; as in pratfall, a vaudeville term; also a term of abuse for a person)