Fallon on Trump's Twitter attack: 'Shouldn't he have more important things to do?' "Melania," Fallon said, "I don't think your anti-bullying campaign is working."

Jimmy Fallon took off the gloves Monday night after President Donald Trump mocked him on and said the comedian should "be a man" and stop "whimpering."

"As you may have heard, last night, the president of the United States went after me on Twitter," Fallon said during his monologue on "The Tonight Show." "So, Melania, if you're watching, I don't think your anti-bullying campaign is working."

Trump slammed Fallon on Sunday for discussing backlash he received after he tussled Trump's hair in an interview during his presidential campaign.

In a Twitter post, Trump wrote that Fallon "is now whimpering to all that he did the famous 'hair show' with me (where he seriously messed up my hair), & that he would have now done it differently because it is said to have 'humanized' me-he is taking heat. He called & said 'monster ratings.' Be a man Jimmy!"

Fallon, who called Trump his No. 1 fan, said he was shocked the president even had time to acknowledge him on Twitter.

"When I saw that Trump insulted me on Twitter, I was gonna tweet back immediately, but I thought, 'I have more important things to do," Fallon said Monday. "Then I thought, 'Wait, shouldn't he have more important things to do? He's the president! What are you doing? You're the president! Why are you tweeting at me?"