Co Sign: A co-sign (or cosign) is a legally binding agreement between two parties where one party agrees to be responsible for fulfilling the terms of a contract if the other party fails to do so. This means that if an individual or business needs a loan but lacks sufficient credit, another individual with better credit can agree to co-sign the loan and take on responsibility if the borrower defaults. Co-signing involves considerable risk, as the co-signer may be liable for the full debt if the original borrower is unable to keep up with payments.