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Suddenly, Travis Barker's got a lot riding on Brock Lesnar vs. Mark Hunt -- if he doesn't pick the winner he's gonna pay off a bet with his body.

We got Travis in NYC and our camera guy offered him a wager on UFC 200's co-main event ... and the Blink 182 drummer simply couldn't refuse.

Check out the video to see who Travis is backing. He says he's actually willing to squeeze a TMZ tattoo onto his already-covered body, IF the wager goes south.