She kicked me in the "sac" or "sack"? Reference is to the testicles

Sack is short for nut sack or nutsack, meaning the scrotum.

(In case it needs spelling out, the allusion is to a bag containing nuts, i.e. testicles.)

A word spelled sac also exists, which Oxforddictionaries.com defines as

A cavity enclosed by a membrane within a living organism, containing air, liquid, or solid structures.

The latter spelling is mainly used in a life-science context.

All due respect, Mr. Kowal, but I have always spelled it nutsac. (And apparently, Coolio, and other artistes of of his genre, agree.)

I have also learned (in researching this particular question) that there is apparently a sport called disc-golf (which really shouldn't surprise me because I went to a hippie college where the only sport was "freestyle frisbee" and the school colors were heliotrope and puce, which are two very similar shades of purple) and the purveyors of a special bag for carrying the equipment (frisbees) for said sport have appropriated the name "NutSac", but failed to register the trademark - leading to the conclusion that too many others regularly use it for another purpose.

Regarding the question, I submit that sac is less ambiguous.