A female having an appearance of such hideous nature that not one, but two sacks are required to facilitate consensual intercourse between the female and a second party. The sacks are referred to formally as the principal and secondary sacks. The principal sack is placed over the head of the aforementioned female and serves as precautionary measure in the prevention of unnecessary visual exposure by the second party. Such visual exposures have previously been observed to cause severe gastric disturbances resulting in projectile fluid discharge--not to be confused with sexual lubricants (unsuitable viscosity). The secondary sack, more commonly known as the "safety net" is placed over the second parties head as a means of N+1 redundancy. Thus, should the principle sack fail (i.e., become banged off), the secondary sack shall become the primary means of visual protection. It should be well noted that neither sack is intended nor capable of preventing pregnancy or STDs.
Sam: Dude, I accidentally banged the principle sack straight off this two sacker the other day.
Nick: Holy shit. Dude you gotta be more careful.
Sam: Yea man, good thing I had my secondary sack stapled to my forehead.
Nick: Thank god for the N+1 rule.