When you totally own someone at something such as a video game, a sport event, or when you have a great come back. Can be said in a normal voice, or yelled excitedly.
Friend 1: *pushed friend 2 off rainbow bridge*
Friend 2: Jerk! I was winning!
Friend 1: Boomroasted.
So if you say something and some one has a come back someone else can say "Ohhhh Boom Roasted!"
Boy" That's what she saidd!!!"
Boy 2" OHHHHHH boom roasted!!!"