What was the sack that Heath Lddger was sewn in in the movie "The Patriot"?

Haven’t seen the movie, but I think what you are referring to is the practice of bundling.

The sack was known as a “bundling stocking” and was part of a puritan courting tradition whereby parents allowed young adults who were courting to sleep together in the same bed, but with various restraints in place such as the bundling stocking in order to prevent things from going too far. This practice allowed the courting couple to ensure that they had chemistry before they married. See Fischer, David H. Albion’s Seed (1989) p79–80 for more info.