What exactly is a “shepherd’s bag”, such as David carried in 1 Samuel 17:40?

The word is of uncertain meaning, but appears to refer to the kind of small bag that shepherds would carry with them, probably containing food and in this case also used for stones to throw.

1 Samuel 17:40 (NET) He took his staff in his hand, picked out five smooth stones from the stream, placed them in the pouch of his shepherd’s bag, took his sling in hand, and approached the Philistine.

This is the NET’s footnote on the word.

48 tn This Hebrew word occurs only here and its exact meaning is not entirely clear. It refers to a receptacle of some sort and apparently was a common part of a shepherd’s equipment. Here it serves as a depository for the stones that David will use in his sling.

Biblical Studies Press (2006). The NET Bible First Edition.