werewolf, were-wolf, n. a human being transformed into a wolf, or capable of assuming a wolf's form or wolf-like characteristics; from Middle English, from Old English werwulf (wer "man" + wulf "wolf"); predates 12th c. Also werewolfism, lycanthrope, loup-garous, lupine. See lycanthropy, garou. [GJ1]

Addendum: European werewolf myths probably started with the berserks, Norse warriors believed by their contemporaries to take on bear- or wolf-like characteristics in battle. [GJ1]