Comparing someone to the results of a particular bodily function is a popular way of expressing disapproval of someone and was no less so in Francis Grose’s day, as we can see from his A Classical Dictionary of the Vulgar Tongue (1785). A shit sack means a “dastardly fellow”, as it does today, although we usually invert the term and insert a preposition. Intriguingly, it was also a term for a Non-Conformist.
Grose launches into a lengthy explanation as to why. After the Restoration of the monarchy, the laws against Non-Conformists were particularly severe and they had to meet in secret and in obscure locations. One group met in a barn and the preacher, lacking a ladder or a tub on which to stand, was suspended in a sack from a beam and addressed his congregation. To add a bit of dramatic impact to his sermon on the last judgment he detailed the terrors of the wicked at the sounding of the trumpet.
Unbeknownst to him, there was a trumpeter from a nearby puppet show hiding underneath the straw in the barn. At the appropriate moment, the trumpeter sounded the charge. “The congregation, struck with the utmost consternation, fled in an instant from the place, leaving their affrighted teacher to shrift for himself. The effects of his terror are said to have appeared at the bottom of the sack, and to have occasioned the opprobrious appellation by which the non-conformists were vulgarly distinguished”.
Now we know!