Singlehearted refers to someone who is devoted, sincere, and committed to a particular cause or person. Its antonyms are words that carry the opposite meaning of singlehearted. Some antonyms for singlehearted are disloyal, treacherous, unfaithful, faithless, insincere, insidious, deceitful, and duplicitous. These antonyms are used to describe people who lack sincerity, fidelity, and dedication. For instance, a person who is unfaithful in a relationship is considered the opposite of someone who is singlehearted. Similarly, someone who is insincere and deceitful cannot be described as devoted or committed. In general, singlehearted people are considered trustworthy and genuine, while the antonyms represent traits that are untrustworthy and fake.