A haversack is a small bag with a shoulder strap. It is nothing like a small backpack in many ways as a small backpack is a small rucksack
with 2 straps. The haversack is mostly hung over one shoulders and is a small square shape. There are some exceptions of course.
It is usual about 10 to 16 inches each side with a a flap that either has straps or clips to close it. It is thought that it derived from the use of bags for carrying oats for feed .This was used by horsemen around the 1800;s and became known as a haversack. In recent years it has become a fashion item.