When expressing a lack of enthusiasm or sincerity, there are several synonyms for the phrase "not wholeheartedly". Some possible terms include halfheartedly, reluctantly, hesitantly, lukewarmly, insincerely, apathetically, and unenthusiastically. Each of these words conveys a slightly different shade of meaning: halfheartedly suggests a lack of energy, reluctance indicates a resistance to the task at hand, hesitantly implies uncertainty or indecision, lukewarmly suggests tepid feelings, insincerely implies a lack of honesty or integrity, apathetically suggests a lack of interest or motivation, and unenthusiastically implies a lack of passion or excitement. Depending on the context, one of these synonyms may be a more accurate way to convey the speaker's lack of enthusiasm.