female genital organ vs fanny

female genital organ and fanny both are nouns.

Word	Noun	Adjective	Verb	Adverb
female genital organ	Yes	No	No	No
fanny	Yes	No	No	No

As nouns, female genital organ and fanny are synonyms defined as:

Other synonyms of female genital organ include female genitalia, female genitals.

female genital organ (noun)	fanny (noun)
external female sex organs	the fleshy part of the human body that you sit on
	external female sex organs