Pete Davidsonsays his grandpa discovered gay porn on his computer, which led to fists flying at the computer monitor.

Check out the clip TMZ obtained … the comedian unloaded the outrageous childhood story during a recent stand-up set … recalling how he accidentally clicked on a graphic gay porn video when he was just 8 years old — and wound up watching the whole thing out of sheer curiosity.

Pete said the “Lemon Party” video featured six 70-year-old men hooking up to the whistling theme song from “The Andy Griffith Show” … which somehow makes the situation even more absurd.

Things spiraled when Pete heard his grandfather coming downstairs after an outing at the Knights of Columbus. Panicking, Pete shut the screen off — but his grandpa wasn’t buying the innocent act and demanded he turn the computer back on.

Pete says he tried warning him … even joking it looked like “the worst night at the Knights of Columbus ever” — but grandpa insisted.

Then came the fatal mistake — Pete says he not only turned the screen back on … he turned the volume up, too … apparently wanting his grandfather to get the “full experience.”

According to Pete, his grandpa heard just a split second of the famous whistle — at the end of the song, and the video — before completely losing it … punching the monitor, smashing the computer, and turning away in horror.

Then came the line that apparently ended the entire family discussion forever: “Peter, we are never to speak of this.”