pay up vs default

pay up is not a noun while default is a noun.

Word	Noun	Adjective	Verb	Adverb
pay up	No	No	Yes	No
default	Yes	No	Yes	No

pay up (noun)	default (noun)
	loss due to not showing up
	an option that is selected automatically unless an alternative is specified
	loss resulting from failure of a debt to be paid
	act of failing to meet a financial obligation

pay up (verb)	default (verb)
cancel or discharge a debt	fail to pay up