“Serenity Now” is a phrase from the Season Nine episode of the same title, “The Serenity Now”. Frank Costanza learns this phrase from a self-help recording. Instead of saying the phrase calmly, Frank yells it loudly, especially when arguing with Estelle.
It turns out the technique is ineffective, according to George's nemesis, Lloyd Braun. Lloyd spent time in a mental institution because he suppressed his own anger for years; he described it as "serenity now, insanity later".
Kramer tries using the technique but explodes anyway, destroying 25 computers that George had been storing in Kramer's apartment. After failing, George's father starts using "Hoochie mama" while Estelle tries to put her car into the garage.