half-slip vs underskirt

half-slip and underskirt both are nouns.

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

As nouns, half-slip and underskirt are synonyms defined as:

Other synonyms of half-slip include petticoat.

half-slip (noun)	underskirt (noun)
undergarment worn under a skirt	undergarment worn under a skirt