Bin Linner was a Sad Sack when here got blown to bits in his secret compound ......I was also the first time he ever got the bullet from a job and in his case he finally got the Point that he was so Dis liked...:)D
I can think of a couple different meaning it might take on. Being the good wizardress I am (cough, cough,) I will go with this: It is a sack that is old and beaten up and never been taken care. It has given its life to keeping your things protected and easy for you to take with you. Then, you turn around and treat it like a piece of beaten burlap. This would be a very sad sack.