brush-off vs rejection

brush-off and rejection both are nouns.

Word	Noun	Adjective	Verb	Adverb
brush-off	Yes	No	No	No
rejection	Yes	No	No	No

As nouns, rejection is a hypernym of brush-off; that is, rejection is a word with a broader meaning than brush-off:

brush-off (noun)	rejection (noun)
a curt or disdainful rejection	the act of rejecting something
	the speech act of rejecting
	(medicine) an immunological response that refuses to accept substances or organisms that are recognized as foreign
	the state of being rejected