The word "co-signed" refers to the act of jointly signing a document, such as a loan or a lease agreement, with another person. When considering antonyms of "co-signed," words like "refused," "declined," "rejected," "disagreed," and "opposed" come to mind. These words signify a refusal or lack of agreement to something, particularly in terms of financial responsibility. Other antonyms may include "uninvolved," "uncommitted," and "unattached," which suggest a lack of participation or responsibility. Ultimately, the antonyms of "co-signed" all point towards an absence of shared responsibility or agreement.