The saying "Roastin" first originated with the black comedian Beetlejuice. He claimed when black people go out in the sun, they don't tan, they "roast." After watching this show that featured Beetlejuice, our group of friends started using the word the way Beetle used it. We would be outside in hot weather (and/or in the sun) and exclaim, "Man, It's Roastin out here." This is where our definition of Roastin was created. So, in summation, "Roastin" is just a cooler way to say "it's hot outside today" or how good looking a girl is.
Man, I'm roastin!
Man, It's roastin out here!
That girl is roastin!
When a person is being extremely witty towards another.
The roastings began when people would honor a person by poking fun at them or their career.
Person1: You smell like hair products.
Person2: Dang, you roastin'.