A humorous insult not particularly referring to anything with complete certainty. Could be referring to the male genitals or something else completely.
Hey, that kid Mark over there thinks he's prettytough. What a sack!
A stretchy skin pouch used to house the testicles. Useful for storing a load until it can be squirted onto a girl's face. Especially if said girl likes the taste of ball sweat.
"hey babe, I heard you like salt and vinegar"
"lol ya it tastes like ball sweat"
"Wanna lick my sack while I jerk off and cum on your face"
"omg really? Will I get a fat load?
"yeah coz you're a dirty skank"
"sweet"
In Iowa this term refers to a plastic bag which you may be offered at your local gas station or grocery store. If anyone ever offers you a sack, the correct response is, "sure, I'll take as many sacks as I can get." A sack can also be used as a pair of shorts in a drunken state.
"Would you like a sack with your busch lights and magnum condoms?"