What is "slap bet"

n. (context US English) A wager between two people (especially children) that involves no money, but in which the winner is allowed to slap the loser as hard as they can.

"Slap Bet" is the ninth episode in season 2 of the popular sitcom How I Met Your Mother. The episode, originally titled "Robin Sparkles", first aired on November 20, 2006. The episode received highly positive reviews and features in several lists of the best How I Met Your Mother episodes in the series. The episode led to two recurring jokes and storylines in the show: "Robin Sparkles", Robin's teenage pop star career, and the Slap Bet, in which Marshall slapped Barney at various points throughout the series.