The opposite of half-hearted is enthusiastic. Other antonyms of half-hearted include wholehearted, warm, eager, passionate, hearty, intense, and ardent. Synonyms of half-hearted include tepid, neutral, unenthusiastic, lukewarm, perfunctory, icy, chilly, and cold. Synonyms of half-heartedness include coldness, heartlessness, imperturbability, callousness, hardness, obduracy, and coolness. ^{merriam-webster.com 1 2 3 4}