The word "co-signed" refers to an agreement to assume joint responsibility for the repayment of a loan or other financial obligation. Some antonyms for co-signed would include words such as "refused," "declined," or "rejected," indicating a lack of willingness to participate in such an agreement. Other antonyms might include words like "released," "discharged," or "absolved," suggesting that a previously assumed responsibility has been lifted. In the context of financial obligations, the term "solo-signed" might also be appropriate, indicating that one party will assume full responsibility for the debt, rather than sharing that responsibility with another party through a co-signature arrangement.