Max Landis had the idea to make a short film, which involved getting people in front of a camera and letting them slap one another, in 2014. One girl took many of her slaps on her bottom. So the first clip is just a supercut of all of those.
Here is the original short: The Slap
Here is an explanation of the project: The Slap: Point of Impact
Here is a post-release follow-up (which is where a lot of the butt slaps come from): The Slap: UltraViolence