The word "co-sign" is often used to refer to a joint signature or endorsement, particularly in the context of loans or financial agreements. Some antonyms for "co-sign" might include "reject," "disavow," "oppose," "refuse," or simply "decline." These words indicate a refusal or lack of support for a particular agreement or proposal, rather than a willingness to endorse or lend support. Depending on the context, other antonyms might include "disagree," "contradict," or "renounce." Antonyms for "co-sign" are useful for expressing disagreement or disapproval, particularly in situations where multiple parties are involved in a decision-making process.