A bag carryed around by bums and hobos (mostly hobos) they are usually full of random junk that the hobos say are "priceless" or "used by (insert famous persons name here)" the hobo sacks are usually plastic bags

Also it can just be a backpack filled with random junk that is not needed. Dose not have to be carried by a hobo. It can be a broken up backpack carried by a middle class person