half-slip vs unmentionable

half-slip and unmentionable both are nouns.

Word	Noun	Adjective	Verb	Adverb
half-slip	Yes	No	No	No
unmentionable	Yes	Yes	No	No

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

Other hypernyms of half-slip include undergarment.

half-slip (noun)	unmentionable (noun)
undergarment worn under a skirt	a garment worn under other garments

half-slip (adjective)	unmentionable (adjective)
	unsuitable or forbidden as a topic of conversation