The 37-year-old actor carries the artificial body part with him to every set in case he's forced to drop his pants on camera, and he admits the bizarre prop came in handy when he was performing his debut episode for 'Two and A Half Men' in 2003.
Speaking on Conan, his co-star Jon Cryer, 49, said: "There was a very clear moment. In the very first episode that Ashton was in, there was a point at which he was supposed to drop trou and be completely naked in front of me.
"We didn't know how to do that in front of the live studio audience without him actually being naked, cause the whole point was, it was supposed to shock people."
He added: "We said, 'We're gonna have to cover him up somehow, but it still has to be shocking
"Everybody was trying, people were working this over in the writers' room, and then Ashton pipes in, 'Oh, I've got a huge prosthetic penis in my trailer!' "
Ashton was clearly unfazed by the revelation and said: "Doesn't everyone?"
He then turned to the comedian and added: "You too can have one made. I had these web shorts that I did for my company and we were producing these various web shorts, like hidden camera things.
"One of the things that we did was this sketch called 'Naked Man'. We had this guy and we had him put on this prosthetic unit, and we sent him to interesting places... I was like, 'Who knows when you're gonna need a prosthetic penis?' So, I kept it and I just happened to have it around."
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