Green’s Dictionary of Slang

(US) a mule; thus a term of abuse.

1845		in G.W. Harris High Times 46: He lied a jassack to death in two hours!
1862		letter q. in Wiley Life of Billy Yank (1952) 96: The man licks the nigers and nigers the jassacks, and in that way drive business.
1894		Wash. Times (DC) 15 Aug. 2/1: The gray matter in his cranium was not of a sufficient strength to prevent him making a jassack of himself.
1898		Daily Public Ledger (Maksville, KY) 3 May 2/2: The highfalutin’ regard to which its eminently Spanish jassackical contemporary is entitled.
1904		Eve. World (NY) 18 May 14/4: He [...] pointed to a jassack browsing on a tuft of curbstone.
1906		(ref. to mid-19C) C.R. Fenno ‘19th century Illinois Dialect’ in AS LVIII:3 250: Why, that’s our jassack, Uncle.
1931		R.E. Howard ‘Breed of Battle’ Action Stories Nov. 🌐 Grieson glared at me, trying to scare me before the scrap started – the conceited jassack.
1944		J. Archibald ‘Defective Bureau’ in Popular Detective 🌐 ‘Can I go?’ Willie asked. ‘I got to die in harness.’ ‘Like any jassack,’ Satchelfoot Kelly said.
1950		P. Highsmith Strangers on a Train (1974) 64: ‘You ol’ jassack!’ one said.