underclothing vs unmentionable

underclothing and unmentionable both are nouns.

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

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

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

Other hypernyms of underclothing include undergarment.

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

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