Matt Barnes Rips Tristan Thompson: You're FAKE Tough!

Matt Barnes says the REAL reason people in the NBA don't like Tristan Thompson is because he's a fake-ass tough guy who's all talk ... no action.

Since Barnes is tight with Draymond Green -- who also hates Tristan -- we asked him where the animosity comes from ... and Barnes didn't mince words.

"A lot of people are tough on the court and then when you seem them off the court, they're not as tough as they want to be," Barnes told us.

Then, he added this shot ... "People know [Tristan's] not built like that. It's known, period."

As for Draymond ... he refused to shake Thompson's hand in Cleveland after the four-game sweep, and then said at the championship parade Tuesday, "Tristan, we ain't cut the same."

Barnes says the beef has nothing to do with the Khloe Kardashian drama ... it's solely about Tristan talkin' tough, but not backing it up.