underpants vs undergarment

underpants and undergarment both are nouns.

Word	Noun	Adjective	Verb	Adverb
underpants	Yes	No	No	No
undergarment	Yes	No	No	No

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

underpants: an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin
undergarment: a garment worn under other garments

Other hypernyms of underpants include unmentionable.

underpants (noun)	undergarment (noun)
an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin	a garment worn under other garments