The term is a derivative of "narb" which stands for a "No Apparent Reason Boner". A narby is typically one who is acting like a mook or turbo; a douchebag.
"That guido over there has a lot of gel in his hair and is wearing an extremely tight black shirt. What an absolute narby."
"Yes, that B&T mook needs to take himself back to Strong Island and drink a large bleach smoothie on the way."