The word "co-signs" refers to the act of jointly signing a document or agreement with someone else. Some antonyms for this word could be "opposes," "disavows," or "rejects." If someone opposes a decision or action, they do not agree with it and will not sign off on it. Similarly, if they disavow something, they deny any involvement or responsibility for it. Lastly, to reject something means to refuse it or not agree to it. These words are all opposite in meaning to "co-sign," as they all imply a lack of agreement or participation in an action or decision.