Is it ensure, insure, or assure?
late Old English werewulf "person with the power to turn into a wolf," from wer "man" (see virile) + wulf (see wolf (n.); also see here for a short discussion of the mythology). Cf. Middle Dutch weerwolf, Old High German werwolf, Swedish varulf. In the ancient Persian calendar, the eighth month (October-November) was Varkazana-, literally "(Month of the) Wolf-Men."