Serenity Now comes from the Seinfeld episode The Serenity Now, where Frank Costanza is told to repeat the phrase to control his blood pressure. Played by Jerry Stiller, Frank turns a relaxation tip from his doctor into repeated shouting whenever he gets angry. The scene became one of the character's most quoted moments and the clip spread online as a reaction sound for stress and impatience. Its popularity lasts because the phrase is simple, memorable, and instantly linked to Frank's explosive temper.