Where does the term “Sack Coat” come from and What does Sack Coat mean?

A sack coat is not (or at least, is hardly ever) made of sackcloth, but, because it is a loosely fitting garment, hanging straight from the shoulders, is so-called simply because it gives the impression of having been fashioned from a sack.

Sackcloth is just that, a coarsely woven cloth from which sacks are made.