The place inside the anus where your excess farts are stored for later in use during the day.
Kyle was storing his farts in his fart sac, then Marc punched him in the stomach and Kyle proceeded to fart, releasing all the farts from his fart sac that he stored.
The bed/couch/etc. where you have been sleeping, lounging, eating, playing video games, watching t.v. etc. for more than a couple of days in a row, without showering or going out. You only get up to use the restroom, get something to eat/drink and then flop back down. This type of situation usually means that there is a definate funk surrounding you that smells something like a fart.
Yo Man, roll outta that fart sack and lets go get some grub!! Shheeeeiit, you been layin up in this fart sack for the past WEEK!!! C'mon, lets roll.