This refers to the supposed transformation of a person into a wolf.
The concept of the werewolf is embedded in European folklore; it must have been triggered by dread of the wolf, the top predator of the region. In other cultures humans were similarly said to change into the most dangerous predator of the area, such as bear, hyena, leopard or tiger.
Ancient stories of man-wolf transformations told by writers like Ovid and Pliny (for example, the legend of Lycaon, who was turned into a wolf by the god Zeus) continued in medieval folklore. Stories told of humans who became wolves at night, and who in that form hunted people, but who returned to human form by day. The transformation, often at the full moon, was sometimes said to be the result of a curse. Lycanthropy was also at times thought to be a kind of witchcraft, the power to metamorphose at will from human to wolf and back having been given by the Devil. There are several cases of supposed witches who were burned at the stake for the crime. Stories of werewolves have been kept alive by writers such as Bram Stoker and by films like An American Werewolf in London and The Howling. The same word also applies to a kind of mental disorder in which a person thinks that he or she can turn into a wolf.
It derives from Greek lukos, wolf, plus anthropos, man. If you want a Germanic equivalent, try the rather rare werewolfery — the first element of that has often been identified as the Old English wer, man, though this is not altogether certain.