Dak Prescott Jokes About Crapping On Eagles, 49ers In Cancer PSA

Dak Prescott is calling on NFL fans to drop trou' and take a deuce all over their hated rival teams ... and it's all in the name of colorectal cancer awareness!!

The Dallas Cowboys quarterback released the hilarious, minute-long promo on Friday ... where he's inviting diehard football backers to join the bowel movement by getting their colons screened and supporting their allegiances at the same time.

"As a professional quarterback, I get a lot of s***," Prescott says in the video for the Colorectal Cancer Alliance's Lead From Behind campaign. "And, I get it -- when you're not a fan of something, s***ing on it can make you feel good. But, what if I told you now, it can do some good, too?"

Prescott then goes on to reveal an at-home screening kit with customizable stickers ... and naturally, the collection features images of pretty much every NFL team's mascots, so people can literally s*** on any fanbase.

"Not a fan of marine life?" Prescott asks as he holds up a dolphin sticker. "Slap it on!!"

Prescott also shades the 49ers, Browns and Eagles ... but conveniently, there's no Cowboys-related sticker in sight.

"Have issues with old-timey prospectors? Boom," Prescott continues. "It works with anything -- from colors to large American predatory birds."

Actor Ryan Reynolds' voice makes a brief appearance, too ... and even he catches a stray in the process.

The clip is pretty damn funny, but the topic is serious for Dak -- he lost his mother, Peggy, to colon cancer in 2013.