long johns vs underclothes

long johns and underclothes both are nouns.

Word	Noun	Adjective	Verb	Adverb
long johns	Yes	No	No	No
underclothes	Yes	No	No	No

As nouns, underclothes is a hypernym of long johns; that is, underclothes is a word with a broader meaning than long johns:

long johns: warm underwear with long legs
underclothes: undergarment worn next to the skin and under the outer garments

Other hypernyms of long johns include underclothing, underwear.

long johns (noun)	underclothes (noun)
warm underwear with long legs	undergarment worn next to the skin and under the outer garments