Upon realizing her gaffe a few seconds later, the 32-year-old singer and now fashion designer covered her mouth in embarrassment, saying, "Oh shoot! Well, I guess I just told the world that I'm having a boy."
Kimmel was completely unaware of Simpson breaking the news on his show until she pointed it out.
"I didn't even know what was happening," he said. "I thought maybe you were having a girl with a penis. I don't know. Well, congratulations."
Now with the word out on the sex of her second child, with whom she announced she was pregnant in December, Kimmel tried to pry more information about her baby boy and asked about his name. Rumors have suggested that she will name her child "Ace," which she didn't refute on the talk show.
"Let's hope it's a boy. Could Ace be a girl?" Simpson addressed the rumors. "I kind of just outed that I'm having a boy, right? I can't believe I did that. That was not planned."
Although this is her second child, Simpson has yet to marry the father of her children, former NFL tight end Eric Johnson. Marriage, she said, will come when she can take a break from pregnancy, as she had her first child in May 2012.
"I keep planning on getting married but I keep getting knocked up," she said with a smile. "It's just one of those things. ...Maybe I should just completely reverse and just stop having sex all together until Eric and I actually say 'I do.'"
Upon returning from a commercial break, the Fashion Star judge once more revealed her regret for making the revelation in such an unrefined manner.
"I just did it so crude too. I feel awful. I talked about wieners."
The Jimmy Kimmel Show airs weeknights at 11:35 on ABC.