Barney (Neil Patrick Harris) lost a bet to Marshall (Jason Segel), and the prize was that Marshall gets to slap Barney as hard as he can five times. Of course, Marshall didn't do all five at once. Instead, he dragged each slap out throughout the course of the series.
Lily (Alyson Hannigan) is Slap Bet Commissioner, so she sets the rules — one of which is that there can be no slaps on Thanksgiving, which foils Marshall's elaborate plan to freak Barney out, which included a website with a countdown clock to when the slap was supposed to happen. But once Barney starts rubbing it in Marshall's face that his countdown would lead to nothing, Lily lifts the slap ban and Marshall sends Barney spinning to the ground... and then performs a song about how wonderful the slap was.
Thus, the story of Slapsgiving was born.
Then, of course, there was Slapsgiving 2, when after the slaps were ruled to be transferable and the rest of the gang had a chance to consider enjoying a slap, Marshall lulled Barney into believing there would be no slap after all:
And Slapsgiving 3, where Marshall was able to have his fourth slap again:
How I Met Your Mother aired from 2005 - 2014 on CBS.
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Andy Hunsaker has a head full of sitcom gags and nerd-genre lore, and can be followed @AndyHunsaker if you're into that sort of thing.