Hang around any group of reenactors and you will always see them with a haversack. Even though most of us throw our car keys and wallet in there, it’s not a purse. It’s an essential piece of military kit that dates back to the Middle Ages. The haversack gets its name from the Dutch word haverzak which means oat bag (haver = oats + zak = sack). This is the bag in which horsemen carried the oats for their horses. Oats were a staple foodstuff of the poor in Europe, and the British would grind them into flour (haver-meal) mix them with water to make a crude bread know as haver-bread. The haversack is a strong bag made of linen or canvas, in which soldiers carry their provisions for the day.
Sailor had similar bags they carried but since they typically did not march long distances from their foods stores these were used to store their spare clothes. These were known as ditty bags. Within the ditty bag were placed several smaller pouches which contained a sewing kit, money, and even souvenirs from their travels. The origin of the name “ditty” is obscure; it may possibly be traced to a cotton cloth known as dutty which was used to make sails.
Trappers and frontiersmen would usually be found with two bags slung across their shoulders: their powder horn and their “possibles bag.” It was so-named either because it contained everything you might possibly need for the day. Typically, the possibles bag was stocked with all kinds of essentials: tobacco and pipe, cup, flints, food, tools, etc. Theses bags were generally quite large and made of animal skin, and either slung over the shoulder or attached to a belt. Most reenactors use their haversacks as possible bags but since we are portraying soldiers, we call them haversacks.
BTW, if you want one of these fine leather possible bags/haversacks, let me know. I sell them for $65. These are completely bespoke so pick your size and color (to a point).
Michael Carver -- Historical Interpreter/Reenactor 