"It's not like it's a dirty little secret," Osbourne, 60, said before telling the story.
"When I was 25, I just moved to America," she explains. "I didn't know many people."
That would have been around 1978, before she had started to date Ozzy Osbourne.
"I had met my husband [Ozzy], but we weren't together. We were just friends. I didn't have a boyfriend. I never did date a lot at that time, anyway, because I was not very dateable."
One night, she went to a club where one of her friends worked.
There, she saw Leno performing his comedy routine. Osbourne told the friend that she thought he was "very nice, very funny. So I said to my friend, ‘I like him.’”
Naturally, the two prank called him. Osbourne kept calling after the prank, too.
"He liked my voice," Osbourne said. "One thing led to another. He came to my house and met me and then we had a little fling. The fling was more fling for me and not fling enough for him," she said, calling it a "flingy-wingy."
Their fling didn't last, though. A few months in, Leno brought around a woman named Mavis -- his future wife, whom he wed in 1980 -- and showed Osbourne around town.
She was lovely and they took me to Fat Burger and they showed me around town," Osbourne said, adding, "They were so kind to me and continued the friendship over the years."