Reynolds and wife Blake Lively welcomed a baby girl in December 2014, and while the actor is thankful that little James is "no longer allergic to sleep," he’s got a new obstacle to deal with.
"She’s saying ‘Mama'," he admitted to Fallon. "What my wife doesn’t realize is [James] calls me ‘Mama’ too. Seriously. I literally had to sit her down and tell her that penises don’t work that way. She understands – she gets it."
But the Deadpool star -- who also stuck around for a round of "Slap Jack" with the late night host -- said that his new role as a father isn’t the only reason he feels uncool these days.
"About a month ago, I was sitting in traffic and I was just jamming to 'These Dreams' by Heart," he recalled. "I was killin' it in the car and there was this busload of high school kids that were parked right beside me. I didn't even realize it. I looked over and every one of their jaws were just on the ground laughing their guts out at me."
"These little high school pricks, you can shove your Nae Nae song up your asses!" Reynolds laughed, before he and Fallon did an impromptu performance. "Twenty years, that’ll be the Nae Nae song, you watch."