hindquarters vs backside

hindquarters and backside both are nouns.

Word	Noun	Adjective	Verb	Adverb
hindquarters	Yes	No	No	No
backside	Yes	No	No	No

As nouns, hindquarters and backside are synonyms defined as:

hindquarters and backside: the fleshy part of the human body that you sit on

hindquarters (noun)	backside (noun)
the part of an animal that corresponds to the human buttocks	the fleshy part of the human body that you sit on
the fleshy part of the human body that you sit on	the side of an object that is opposite its front