Loyal and sincere all the way through — someone whose feelings and promises are real, not for show. It is an old-fashioned, warm word, the kind you meet in old songs, poems, hymns, and stories about soldiers, friends, or lovers, much more than in speech today. If someone says it now, they usually mean it a little grandly, or with a smile at how old the word sounds. It describes a person (or a person's heart), never a fact or a story — for that you would say 'true'. The feel is not just 'this person will not lie to me' but 'this person will stay with me'; it mixes honesty and faithfulness together in one word. Usually written with the hyphen; you may also see 'truehearted' as one word, especially in older American books.