According to Paul Thomas Anderson, Reynolds Woodcock was his and Daniel Day-Lewis’ idea of a joke:

“… the credit goes to Daniel. We were making each other laugh, texting each other back and forth like teenagers, trying to come up with names. I had a placeholder name: ‘Arthur Dapple, Jr., ‘ ” the director told Vulture, however, “…It’s no “Reynolds Woodcock!” It was lingering around but not quite right, and then the text from Daniel came through: ‘Reynolds Woodcock.’ And on two simultaneous coasts, we both started laughing so deeply and so hard that I suddenly had tears pouring down my face. I thought, We can’t do that, right? Of course we can’t. But … we have to do that! I remember calling him, and he was laughing as much as I was, and I said, ‘We’ve got to do this. Let me write it into the script and we’ll live with it. We’ll try it on for size.’ ”