In a manner that is firmly bound or secured by hand, or with steadfast commitment and reliability.
From 'handfast' (Old English 'handfæst'), meaning to clasp hands in agreement or to bind securely. The suffix '-ly' converts it to an adverb form indicating the manner of action. The word evolved from Germanic roots where 'hand' + 'fast' (meaning 'held' or 'bound') created a term for a solemn pledge made by clasping hands.
Medieval couples sometimes practiced 'handfasting'—a trial marriage where they clasped hands for a year and a day—so 'handfastly' originally meant doing something with the gravity and commitment of that serious handshake ritual, even before formal church weddings existed.
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