The phrase comes from the Seinfeld episode The Serenity Now, first aired in 1997 during the show's final season. Frank Costanza is told to repeat the words to control his anger and blood pressure, but he and later George use them by shouting instead of relaxing. The joke spread because it perfectly sums up forced calm, and the clip has since become a shorthand reaction online for stress, irritation, and pretending to be composed while clearly not coping at all.