Jon Stewart -- Powerslammed By John Cena ... Payback's a Bitch!

Good luck finding your moment of zen now ...

Jon Stewart -- whose old job was safely positioned behind a desk, never once attacked by a musclebound thug -- was powerslammed by John Cena last night ... hard.

The former "Daily Show" host appeared on WWE's "Monday Night Raw" where he was confronted by Cena ... who was still pissed at Jon for hitting him with a steel chair at SummerSlam this weekend.

They argued. Cena got mad. And then picked up Stewart ... twirled him around like a rag doll ... and then punished him with his famous finishing move, the Attitude Adjustment.

Fake or definitely fake ... getting thrown to the mat like that has GOT to hurt.