underclothes vs unmentionable

underclothes and unmentionable both are nouns.

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

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

underclothes: undergarment worn next to the skin and under the outer garments
unmentionable: a garment worn under other garments

Other hypernyms of underclothes include undergarment.

underclothes (noun)	unmentionable (noun)
undergarment worn next to the skin and under the outer garments	a garment worn under other garments

underclothes (adjective)	unmentionable (adjective)
	unsuitable or forbidden as a topic of conversation