The Britain's Got talent judge is on The Lateish Show with Mo Gilligan on Channel 4 tonight and was asked by an audience member if he'd ever seen his bosses' private parts.
Host Mo asked if he had sneaked a peak at the private parts of Simon, 61, when they were both at a urinal together. David, 49, confirmed he had - and the eyeful made quite an impression on him.
Mo said: "So tell us, have you ever had a peek? Tell us about it. We wanna know!"
David replied: "It’s quite chubby. It’s a big one."
Mo seemed quite exciting by the revelation and said: "He’s got a chubby one?!"
David then waxed lyrical, saying: "It was nice. It was a glimpse! I was looking at something else and then ‘Ooh! There it is’ and then I looked away again.
"It’s like looking at the sun, you can’t look at it for too long. I had just a little look."
David also revealed hilarious details of the wedding reception of BGT host Ant McPartlin, 45, who married Anne-Marie Corbett on Saturday at St Michael's Church in Heckfield, Hants.
In one section of The Lateish Show he also ended up stripping off to the delight of the audience.
He also teased viewers with more news about the upcoming collaboration with former Little Britain co-star Matt Lucas.
David Walliams says he had sneaked a peak of Simon Cowell's private parts - and the eyeful made quite an impression on him