The line comes from Seinfeld, where Frank Costanza, played by Jerry Stiller, adopts Serenity Now as a calming mantra to keep his temper and blood pressure under control. The joke is that he shouts it instead of saying it calmly, and the phrase became one of the show's most quoted outbursts after the episode aired in 1997. Clips of the yell later spread through Seinfeld compilations, reaction videos and meme edits, where it is still used as shorthand for forced calm and sudden frustration.