undergarment vs unmentionable

undergarment and unmentionable both are nouns.

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

As nouns, undergarment and unmentionable are synonyms defined as:

undergarment (noun)	unmentionable (noun)
a garment worn under other garments	a garment worn under other garments

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