According to this random website, "fitballing" is when you "head on over to the beach, dig a big-ass hole in the sand, blow up your fitness ball, and bury it about three-quarters of the way in sand. Then you do flips 'n shit off of it! :D." Could have done without that smiley, but you know, I learned something today.
Anyway, now that you know what fitballing is, you've got to see it in action, right? It's certainly something with the flips and gymnastics and everything. McKayla Maroney would be kind of impressed.
(Skip the first minute unless you want to see a bunch of shirtless bros not fitballing.)
[H/T: Reader Sonjay]