Similar to the usage of "Burn", Boom Roasted is said after an insult or "dis"s for emphasis. Coined during the Season 5 episode of The Office titled, "Stress Relief".
When you get boom roasted, it is equivalent of getting burned!
So if you say something and some one has a come back someone else can say "Ohhhh Boom Roasted!"
Girl "Can i play with your ball?"
Boy" That's what she saidd!!!"