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?"
-"Sure, I'll take as many sacks as I can get."