The word "twotimer" is used to describe someone who is disloyal or unfaithful to their partner. Some common antonyms for this word include faithful, loyal, devoted, steadfast, and trustworthy. These words all relate to being consistent and dedicated in one's relationships, which is the opposite of being a "twotimer." Other antonyms could be honest, truthful, or sincere, which refer to someone who is upfront and transparent in their relationships. Overall, using antonyms for "twotimer" highlights the importance of fidelity and honesty in personal relationships.