most commonly used in the UK, the "PROPER" before "BRUTAL" is used to emphasize how BRUTAL something may be (ie.:a show, a band, a music genre).
This term was first heard by most Americans from front man Oliver Sykes from Bring Me The Horizon.
Dick: I've seen four girls today with the MySpace name of "Proper Brutal"
Dick: Oh yah, forgot you were always on Oli Sykes nuts you creep.