Hello, XiaoDeng. You are correct. "Sack" is referring to courage. To be honest, this is not a phrase that is commonly used (at least not in the United States). In fact, it's the first time I have heard it. It would be more common to say nerve, guts or courage. Good question.

Thanks you two, Gary and Jonny.

Johnny's comment is the right answer. It's referring to a scrotum.

This is slang... “sac” refers to a man’s “ ballsac” (man’s privates). It’s the same way as saying “You didn’t have the balls...” which means “You weren’t man enough...”. Yes, it means ”you don’t have the guts” but it is said in a much harsher way for effect. There is the additional element of saying he isn’t brave AND insulting him for not being a man (a man should behave in a certain way).