Oh is that not socially acceptable?
I think the origin of hand shaking was to demonstrate that you're friendly and unarmed. Slapping someone in the butt would be like saying "I'm oh-so friendly and oh-so armed... in my pants."
theres an ATHLETIC butt slap?
I do it all the time. It's not weird at all.
adding to that, the silent custom of two people trying to see who has the better grip... and it gets more awkward