half-slip vs undergarment

half-slip and undergarment both are nouns.

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

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

Other hypernyms of half-slip include unmentionable.

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