The act of eking one's seed out from the testes. Ejaculating. Spunking up a storm in a lady's teacup. Cleaning one's pipes.
Farquar's hand was a blur as he frantically, desperately reached the point of no return. "By Jove, I think it is time to empty one's sack" he ejaculated, plum firmly in his mouth, and with a gruff grunt of pleasure, he emptied his sack onto the shiny, firm booty that was splayed before him.
So Jane made me sit next to her hot friend lisa then sat on the other side of me so I was in the middle, it was the straw that stroked the camel's sack!
This is an expression used to denote something that is irrelevant, useless or unfit for purpose. If you wanted to say someone was stupid, you could say “Her head is as empty as a eunuch’s ball sack.” If you wanted to say someone was useless, “He’s as much use as a eunuch’s ball sack.” Cowardly “If courage was balls you’d be a eunuch’s ball sack.” The expression is versatile and can be adapted to a wide variety of purposes.