knickers vs plus fours

knickers and plus fours both are nouns.

Word	Noun	Adjective	Verb	Adverb
knickers	Yes	No	No	No
plus fours	Yes	No	No	No

As nouns, plus fours is a hyponym of knickers; that is, plus fours is a word with a more specific, narrower meaning than knickers:

knickers: (used in the plural) trousers ending above the knee
plus fours: men's baggy knickers hanging below the knees; formerly worn for sports (especially golf)

Other hyponyms of knickers include britches, buckskins, trunk hose.

knickers (noun)	plus fours (noun)
(used in the plural) underpants worn by women	men's baggy knickers hanging below the knees; formerly worn for sports (especially golf)
(used in the plural) trousers ending above the knee