In the "John Wick" films, criminals fear John Wick by the nickname Baba Yaga.
The name is introduced in the first film when crime boss Viggo Tarasov explains that Wick was once his most terrifying enforcer. Viggo describes him not as an ordinary assassin but as the person sent to kill the boogeyman. That reputation comes from Wick's legendary precision, endurance, and willingness to complete seemingly impossible assignments.
Baba Yaga is a supernatural figure from Slavic, especially Russian and Eastern European, folklore. She is commonly portrayed as a dangerous witch or forest-dweller, sometimes living in a hut standing on chicken legs. The films use the name as a symbol of fear, although folklore specialists and Russian speakers often point out that Baba Yaga is not literally the generic Russian word for the boogeyman.
That translation issue creates a common trivia trap, but it does not change the answer within the franchise: Baba Yaga is the nickname spoken and used for John Wick. He is also called the Boogeyman and the Devil in different franchise contexts. John Wick is portrayed by Keanu Reeves, while the character's birth name is given as Jardani Jovonovich.