A water filled bag used to add ballast to boats in order to produce a larger wake. This enables wakeboarders to get more air.
The Fat Sac ads up to 600 pounds of weight to your boat.
by TT September 12, 2003
a large sac of marijuana.
"Let me buy that fat sac off you."
by tha dude December 17, 2006
Its a sac of fat that is someplace on your body you don't want it
you're a fatsac
by The gr 6 class of holy name May 28, 2011