long johns vs underclothing

long johns and underclothing both are nouns.

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

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

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

Other hypernyms of long johns include underclothes, underwear.

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