The area of a woman's bikini or panties where the labia majora are clearly defined. This differs from a "camel toe" that creates a "w" shape, visible from the front only. The meat sack is more of a "u" shape and is visible from the front, back, or below. The meat sack is best viewed from behind when a woman wears a thong.
1) She's got a nice, tight meat sack.
2) Check out that girl in the pink bikini bending over- you can see her meat sack!
3) I was behind this hot blonde in yoga today, and every time she did downward facing dog, I caught a glimpse of her meat sack.