a grain sack: the large open-topped bag that carried food home on a donkey, used in the Bible only of the bags Joseph's brothers brought back from Egypt
All fifteen uses of this word sit inside one story, the journeys Jacob's sons make down to Egypt to buy grain during the famine. The bags are big. Joseph's steward is told to fill them with as much food as the men can carry, and when the brothers are stopped on the road home each of them has to lower his own bag off the donkey to the ground before it can be searched. The opening at the top is called the bag's "mouth", and that is where the silver keeps turning up, sitting right where the first hand inside will find it.
The word carries the plot. Twice the brothers' silver is put back into these bags without their knowing, and the second time a silver cup goes in as well, so the very bags meant to bring food home become the thing that traps them. By the time the steward searches the brothers in order from oldest to youngest and the cup comes out of Benjamin's bag, the reader has heard the word a dozen times and knows what is coming.
This is not a technical name for some special kind of container. Genesis 42:27 calls the same bag by the ordinary Hebrew word for a sack (שַׂק) at the start of the sentence and by this word at the end of it, and the chapter can also speak of the men's containers with the all-purpose word for gear or vessels (כְּלִי). What sets this word apart is not the bag itself but the fact that Hebrew reaches for it here and nowhere else in the whole Bible.
The name is usually traced to a verb meaning "to stretch out" (מָתַח), the word Isaiah uses of God stretching out the sky like a tent. That fits a bag pulled wide open and filled.