Notorious practical joker WOODY HARRELSON can’t stop bragging about a pisser of a prank he pulled on co-star MATTHEW MCCONAUGHEY during filming of “True Detective,” their new smash HBO hit.

“The guys were pranking each other from Day One, but Woody kicked things off with a diabolical gag,” said an insider.

“He stretched Saran Wrap over the toilet seat in Matthew’s dressing trailer, pulling it so tight it was nearly impossible to detect. So when Matthew went to relieve himself…SPLISH, SPLASH!…he was takin’ a bath! Next, Woody waxed the belt on Matthew’s treadmill, which made him slip, slide and crash next time he jogged!”

Then it was Matthew’s turn! “He sprinkled itching powder in a wig Woody wore for a scene, driving him flat-out batty!”

But Woody got the last laugh! He’d ordered a special director’s chair with Matthew’s name on it, and during a break in filming, called a meeting with his co-star and their director to discuss an upcoming scene. As Woody waved his buddy into his special seat… POW!

“There was the sound of a loud explosion – and the chair collapsed, throwing Matthew to the floor! Everyone jumped out of their skin – until Woody started roaring with laughter.” Said my source: “As Matthew was helped up by Woody, he admitted his co-star had outdone himself. But as they left the set that evening, Matthew whispered to Woody: ‘Sleep with one eye open. It ain’t over yet!’”