(games) A game or activity in which one or a group of players try to keep a footbag off the ground using only the feet.
The foot bag used for such a game.
Etymology
A trademark of Wham-O. It uses reduplication of the word sack with the word "hacky"; a "hack", in the game's terminology, is akin to a point, and happens when every player in a group successfully passes the foot bag.