From Proto-Balto-Slavic *wilkás, from Proto-Indo-European *wĺ̥kʷos. The word would originally have been a descriptive nickname ('the killer, the plunderer, the destroyer'), the original name having perhaps become a taboo word. Note that similarly formed nicknames for “wolf” still occur in modern Latvian: pelēcis (“gray one”), mežainis (“forest one”), mežavīrs (“forest man”), vecbrālis (“old brother”), etc.