Stern brought up the Brandi tryst to the dreamy Scotsman, who explained the circumstances behind his roll-in-the-hay with the Real Housewife, which she rated "11 out of 10."
He said he crossed paths with Brandi "at a beach party," and "had a little bit of fun" with the mother-of-two. He said in casual conversation, he asked her what she did for a living, but she changed the subject in the heat of the moment.
Watch the video on RadarOnline.com
The 300 star said his "Who's Brandi Glanville?" comment came when a pap ambushed him and asked him about the hook-up after she publicly bragged about it.
"You know what I said -- 'Who's Brandi Glanville?' -- because I didn't know her last name, didn't know she was a celebrity, didn't know she was gong to run out and f*cking tell everyone about it!"
Butler's next film, Olympus Has Fallen, hits theaters on Friday, March 22. The Howard Stern Show airs weekdays on SiriusXM.