The word "unfasten" is a transparent combination of two native English elements. The prefix "un-," with its ancient Germanic roots, is one of the most common and productive means of expressing negation or reversal in English. It is attached to the verb "fasten," which itself derives from the Old English adjective "fæst," meaning firm or fixed, a concept traceable to Proto-Germanic. The logic of the word is entirely compositional: to "fasten" is to make secure, and to "unfasten" is to perform the reverse action, to release or open that which was secured. This straightforward morphological process illustrates how English readily creates new verbs to describe precise, opposite actions.

Structure: un (not, reverse) + fasten (make secure) "un-" is a prefix of Old English origin, meaning "not" or "the opposite of an action." "fasten" is a verb of Old English origin, from 'fæstnian', meaning "to make firm or secure."