"Mundian to Bach Ke" (English: Beware of the Boys) is a song originally performed by Punjabi artist Labh Janjua.
It has since been remixed by British artist Panjabi MC.
This song is typically associated with Vikkstar123 among the fans and followers of the Sidemen. Some have even started referring to it as "Vik's Theme" because - due to his Indian descent - other members of the Sidemen have used this music whenever Vik features in their YouTube videos.
This song is used on Vik's 'THE END - SIDEMEN DISS TRACK REPLY', in which he "disses" Deji.
The song is also the theme for The Dictator, a 2012 comedy film starring Sacha Baron Cohen. This is most likely where the Sidemen discovered it.