Green’s Dictionary of Slang

1. a nonconformist [the term was euphemized in 19C as shick-shack (also shig-shag; sic-sac; shuck-shack; shiff-shack etc). Orig. a term of abuse for people who were found not wearing the customary oak-apple or sprig of oak before noon on Royal-oak Day (29 May, commemorating Charles II’s hiding in an oak tree). Such people would most likely be nonconformists or Puritans. That day became known in dial. as Shick-sack Day and the oak-apple or sprig of oak became known as shick-shack. If you wore your oak sprig after noon you became a shick-shack, a fool].

1756		W. Toldervy Hist. of the Two Orphans III 18: I meun them as bin agenst church and stete, bin a parcell a oaves [...] Then you bin wot I took ya for at first, said the shoemaker, that is an oaf, or a sh-t-sack.
1796		Grose Classical Dict. of the Vulgar Tongue (3rd edn) Sh-t sack. [...] a non-conformist. This appellation is said to have originated from the following story:—After the restoration, the laws against the non-conformists were extremely severe. They sometimes met in very obscure places: and there is a tradition that one of their congregations were assembled in a barn [...] where the preacher, for want of a ladder or tub, was suspended in a sack fixed to the beam. His discourse that day being on the last judgment, he particularly attempted to describe the terrors of the wicked at the sounding of the trumpet, on which a trumpeter to a puppet-show, who had taken refuge in that barn, and lay hid under the straw, sounded a charge. The congregation, struck with the utmost consternation, fled in an instant from the place, leaving their affrighted teacher to shift for himself. The effects of his terror are said to have appeared at the bottom of the sack, and to have occasioned that opprobrious appellation by which the non-conformists were vulgarly distinguished.
1811		Lex. Balatronicum [as cit. 1796].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. (also sack of shit) a general pej., an unpleasant person.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Sh-t sack. A dastardly fellow.
1811		Lex. Balatronicum.
1968		A. Burgess Enderby Outside in Complete Enderby (2002) 302: Some shitsack’s been on the jabber.
1986		L. Heinemann Paco’s Story (1987) 126: Even the company shitsack got his ashes hauled while we were there.
1986–7		Maledicta IX 160: Capt. Grose’s shit-sack (‘a dastardly fellow’) is obsolete but could be usefully revived.
1994		T. Willocks Green River Rising 39: I’m warnin’ you, Abbott, you long sackashit.
1998		J. Lansdale Rumble Tumble 113: ‘Goodbye, shit sack,’ Brett said.
2013		F. Bill Donnybrook [ebook] ‘You two shit-sacks take off your clothes’.