The quality of being firmly committed, dependable, or bound by a solemn agreement made by clasping hands.
Formed from 'handfast' plus the suffix '-ness,' which creates abstract nouns describing qualities or states. 'Handfast' traces to Old English and Germanic origins where hand-clasping formalized agreements, contracts, and betrothals before written documents were common.
In medieval times, your word—sealed with a handclasp—was your legal contract, so 'handfastness' was literally a person's reputation for keeping agreements, making it one of the most valuable character traits you could possess in a society without written contracts.
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