step-in vs underpants

step-in and underpants both are nouns.

Word	Noun	Adjective	Verb	Adverb
step-in	Yes	No	No	No
underpants	Yes	No	No	No

As nouns, underpants is a hypernym of step-in; that is, underpants is a word with a broader meaning than step-in:

step-in: short underpants for women or children
underpants: an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin

step-in (noun)	underpants (noun)
short underpants for women or children	an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin