underskirt vs half-slip

underskirt and half-slip both are nouns.

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

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

Other synonyms of underskirt include petticoat.

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