In a sincere, genuine, and deeply felt manner; with complete honesty and warmth from the heart.
From 'heart' (Old English 'heorte') + 'full' (Old English 'full') + '-ly' (adverbial suffix). The combination emerged in Middle English to describe actions done with genuine emotional commitment rather than mere surface compliance.
Before modern psychology, people literally located emotions in the heart—so 'heartfully' literally meant doing something from your actual organ of feeling, making it one of the oldest ways we tried to describe authenticity and sincerity.
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