A roast can be defined as a well timed and smart comeback to an insult or diss that is not too off-topic and usually emotionally hurts the perpetrator
Person 2: I threw a frisbee and it came back and hit me
Person 1: No one cares.
Person 2: About you they don't. (Roast)
Example 2
Person 2: My cat died today
Person 1: Did i ask?
Person 2: But bitches donât ask they listen (Roast)