Wiktionary
n. 1 (context games English) A game or activity in which one tries to keep a footbag off the ground using only the feet. 2 The foot bag used for such a game.
Wikipedia
thumb|right|200px|A Hacky Sack Hacky Sack (also known as Hackey Sack) is the trademarked name of a type of footbag.
The traditional game is played when two or more players stand in a circle and try to keep the sack (or footbag), off the ground. The game starts when one player picks up the sack and tosses it to the chest of another player, who allows it to fall to his feet so he can kick it, and play begins. Play continues until the sack falls to the ground, then a player picks up the sack and the game resumes. The object of the game is to keep the sack off the ground for as long as possible. If every player gets a touch to the sack before it hits the ground, it is called a 'hack'. If every player gets two touches before the sack hits the ground, it is called a 'double-hack' and so on and so forth. The name Hacky Sack has become a genericized trademark, and can refer to either the sack (or footbag) itself or a footbag game. Some common tricks include stalls, bag daggers, lifts, loops, ham spalts, and milk tosses. Categories of more advanced tricks include pixie moves, blurry moves, paradox moves, and gyro moves. It was seen being played as far back as 1955 in the movie To Catch a Thief.
Usage examples of "hacky sack".
As naptime approached, Charlie looped through the neighborhood and headed up through Washington Square Park, where people were reading and lounging in the shade, a guy played guitar and sang Dylan songs for change, two white Rasta boys kicked a Hacky Sack around, and people were generally settling in for a pleasant and windless summer day.
Besides the Frisbee players, there were people playing touch football and a group kicking around a Hacky Sack.