underpants vs unmentionable

underpants and unmentionable both are nouns.

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

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

underpants: an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin
unmentionable: a garment worn under other garments

Other hypernyms of underpants include undergarment.

underpants (noun)	unmentionable (noun)
an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin	a garment worn under other garments

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