A sack fart is when someone farts on your ballsack to create a pleasant warm sensation. The giver of the fart may be male or female, and this process is not neccesarily sexual.
Please note, a sack fart is only present when you can smell the fart with your nose pressed to the ballsack. When the smell expires, the ballsack loses its' title of the sack that doth hold the fart.
*Person 1 farts on the Person 2's sack*
Person 2 : Oh, that's so warm and nice, you're an expert sack-farter.
Person 1 : Why thank-you, I am quite experienced. Enjoy your sack fart while you can, because it will be gone soon!