handfast comes from Old English handfæsten or Old Norse handfesta ('to strike a bargain by joining hands'), a compound of hand and fast ('to make firm, fasten'). Morphologically, the word is best read through the components named in the history, with the modern spelling preserving a compound structure rather than a single unanalyzable stem. In present English, the noun sense is "a betrothal or marriage sealed by clasping hands; to pledge thus", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as hand and fast, and in derivatives such as handfasting. Its semantic neighborhood includes betroth, pledge, engage, and plight, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English hand and Old English fæst.