A person who accepts joint responsibility for the debt of another(noun)
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Use "cosigner" in a sentence
"Normally, a cosigner is required, and a one or two month rental deposit can be applied to the last month's rent."
"Beware: A cosigner is a co-borrower, equally obligated to repay the loan."
"My experience (not necessarily yours) is that CROWN, the international freight broker/freight forwarder, would be responsible for your shipment from FL to the FINAL destination, as the "cosigner" and carrier of your goods."