skivvies vs underclothes

skivvies and underclothes both are nouns.

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

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

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

Other hypernyms of skivvies include underclothing, underwear.

skivvies (noun)	underclothes (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