There are four main categories of when one would say, "roast on you."
1) they are flirting or be flirted with
2) somebody just told them off, they got burned, or they just said something stupid
3) they burned some one else
4) something amazing or extremely hilarious just happened
Girl: I really love you!
Guy: Your best friend told me that too.

Me: Roast on you!
by youknowwwhoitis December 8, 2014
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Something your fellow classmates or peers say before they proceed to roast your existence usually happens if you try to roast them but fail.
Dude 1: Boy your hairline look like scratch art!
Crowd: Cricket
Dude 2: BOY DON'T LET ME GET THE ROAST ON YOU with your high waters looking ass boy mike wazowski sounding voice ass boy
Crowd: OOOOOHHHHHH!!!!
by Dankness at its finest July 10, 2017
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When You roast someone like a muffin it means you roast them Like a white person. (White People can't roast, Besides I'm White, I know what it's Like).
Man 1: Boy, Ima roast you like a muffin!
Man 2: Oh No...
Man 1: Yo mama So fat, She turned into a Pig!
Man 2: *Facepalm*
by TheDarkJester69 April 25, 2016
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December 12th-

National Sam Roasts you day. Sam got roasted yesterday due to December 11th being national roast Sam day. Now it's time for Sam to roast you back. Today marks the day where Sam can roast anyone not named Sam.
Example-

Sam: you're dumber than bricks
Person 2: wha-
Sam: it's national Sam Roasts you day, this is payback for yesterday's roast Sam day
by NullTheFloof December 12, 2020
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December 12th-

National Sam Roasts you day. Sam got roasted yesterday due to December 11th being national roast Sam day. Now it's time for Sam to roast you back. Today marks the day where Sam can roast anyone not named Sam.
Example-

Sam: you're dumber than bricks
Person 2: wha-
Sam: it's national Sam Roasts you day, this is payback for yesterday's roast Sam day
by NullTheFloof December 12, 2020
Get the National Sam Roasts You Day mug.
December 12th-

National Sam Roasts you day. Sam got roasted yesterday due to December 11th being national roast Sam day. Now it's time for Sam to roast you back. Today marks the day where Sam can roast anyone not named Sam.
Example-

Sam: you're dumber than bricks
Person 2: wha-
Sam: it's national Sam Roasts you day, this is payback for yesterday's roast Sam day
by NullTheFloof December 12, 2020
Get the National Sam Roasts You Day mug.