The word 'unnerve' is a relatively modern English formation, first appearing in the early 17th century. It is constructed from the native English negative prefix 'un-' and the noun 'nerve,' which entered English via Old French from Latin nervus. In its original Latin sense, nervus referred literally to a sinew or tendon, the physical source of bodily strength. By metaphorical extension, it came to signify vigor, force, and, crucially, the mental fortitude or courage required for action. Therefore, the literal construction of 'unnerve' means "to deprive of nerve or strength." The logic of its evolution is clear: it shifted from the concept of removing physical vigor to specifically describing the act of undermining someone's psychological composure or confidence, leaving them feeling weak or agitated.

Structure: un (not) + nerve (courage, strength) - un: A prefix of Germanic origin (Old English un-) meaning "not," "opposite of," or "reversal of an action." Its functional role is negation or reversal. - nerve: From Latin nervus, meaning "sinew, tendon; string of a musical instrument; vigor, force." In this compound, it functions as the core noun meaning "courage, mental strength, or steadiness."