knickers vs underpants

knickers and underpants both are nouns.

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

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

knickers: (used in the plural) underpants worn by women
underpants: an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin

knickers (noun)	underpants (noun)
(used in the plural) underpants worn by women	an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin
(used in the plural) trousers ending above the knee