nudity vs birthday suit

nudity and birthday suit both are nouns.

Word	Noun	Adjective	Verb	Adverb
nudity	Yes	No	No	No
birthday suit	Yes	No	No	No

As nouns, birthday suit is a hyponym of nudity; that is, birthday suit is a word with a more specific, narrower meaning than nudity:

Other hyponyms of nudity include nude, altogether, raw, undress, bareness.

nudity (noun)	birthday suit (noun)
the state of being without clothing or covering of any kind	informal terms for nakedness