n: 1)a small sac like ball which is filled with tiny pieces of rock, sand, or plastic.
2) a circle game of usually 4 or more people in which you use a hacky sac to play.
Approximately a bit larger than a golf ball, it is played by using your feet (sometimes hands but only in attempt to keep it in the circle). You can also use your body (such as your head) if the hacky sac is high in the air after a pass.
see also: hack, Biff Master Flex
Julie: Hey guys wanna play some hacky sac at lunch?
Kyle: Yeah sure, I'll bring Howard and a couple other guys.
Julie: Nice! I bet you any money Howard's gunna do some crazy stunts and biff like a mother fucker...as always.
Kyle: Yeah most likely, Oh, don't forget to bring a hacky sac too.
Julie: But of course!