skivvies vs underclothing

skivvies and underclothing both are nouns.

Word	Noun	Adjective	Verb	Adverb
skivvies	Yes	No	No	No
underclothing	Yes	No	No	No

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

skivvies: (used in the plural) men's underwear consisting of cotton undershirt and underpants
underclothing: undergarment worn next to the skin and under the outer garments

Other hypernyms of skivvies include underclothes, underwear.

skivvies (noun)	underclothing (noun)
(used in the plural) men's underwear consisting of cotton undershirt and underpants	undergarment worn next to the skin and under the outer garments