cosigner vs cosign

cosigner is a noun but cosign is not a noun.

Word	Noun	Adjective	Verb	Adverb
cosigner	Yes	No	No	No
cosign	No	No	Yes	No

cosigner (noun)	cosign (noun)
one of two or more signers of the same document (as a treaty or declaration)
a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)

cosigner (verb)	cosign (verb)
	sign and endorse (another person's signature), as for a loan
	sign jointly