Co-signing is generally considered as an act of agreeing with or supporting a proposal, an idea, or a person. However, its antonyms would be disapprove, disagree, disavow, disclaim, or oppose. These words signify the act of opposing or criticizing something, rather than supporting or agreeing with it. By disapproving, a person does not provide any support or agreement to the proposal, while by disagreeing, the person simply has a different point of view. Disavowing and disclaiming indicate the act of denying any responsibility or involvement in something, which completely negates the idea of supporting or agreeing. Finally, opposing means actively working against or resisting something, indicating the exact opposite of co-signing.