SHIRTLESS UNDIES DANCING, BABY!!!

Dallas beat the hell out of Philadelphia in an absolute must-win game Sunday night ... and for the party Monday, their backup center, Joe Looney, got graphic.

The big fella -- measuring in at 6'3", 315 pounds -- stripped off his shirt AND his pants ... and gave us all a pose-off and a jig that were truly epic.

Looney's known as the team's class clown ... and his Monday victory vid definitely didn't do a damn thing to dispute that!!!

Of course, Looney and the 'Boys have every reason to celebrate ... their win over the Eagles puts them at 4-3 and in sole possession of first place in the NFC East.

It's a bye for the Cowboys next week ... but they meet the Giants on "Monday Night Football" in their next contest on Nov. 4.

So, what's Joe got planned for that Tuesday if Dallas comes down with another W???