half-slip vs petticoat

half-slip and petticoat both are nouns.

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

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

Other synonyms of half-slip include underskirt.

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