“Jassack” (or “jass-ack”) is a metathesis of the word “jackass.” The word “jassack” was printed in the Marshall County Republican (Holly Springs, MS) on September 21, 1839.
 
American politician Abraham Lincoln (1809-1865) wrote about “jass-ack” some time in the 1840s, before he became U.S. president.
 
“Jassack band” has been cited in print since at least 1914, and the “jassack” term might have evolved into the words “jass” and “jazz.”
 
   
Chronicling America
21 September 1839, Marshall County Republican (Holly Springs, MS), pg. 3, col. 1:
PREMIUM JASSACK!—The OWNER of the Editor of the “LOAFER” expects to get a premium for him at the approaching cattle show in Tennessee—said editor being “the biggest, ugliest, and loudest braying Jassack in all the country.”
       
Chronicling America
26 September 1840, The North-Carolinian (Fayetteville, NC), “Squire Blumpy, of Rackensack,” pg. 4, col. 1:
A short time since he obtained a commission for one of those tools known in these “diggins” as Jassack” of the Peace, or in other words, Justice of the Peace.
 
22 October 1842, Lexington (MS) Union, pg. 2, col. 1:
DO YOU WANT A HORSE?
Our market is supplied with a perfect cord or horses, mules, and a few jassacks, they can be purchased at low prices.
   
Chronicling America
5 November 1853, Monongalia Mirror (Morgantown, WV), “The Post-Office,” pg. 2, col. 5:
... instead of the (I had like to have said Jass Ack) that now holds it.
 
Google Books
The Widow’s Son; or, Left Alone
By Emma Dorothy Eliza Nevitte Southworth
Philadelphia, PA: T. B. Peterson & Brothers
1867
Pg. 348:
“This old jassacks knows it all!” Amy smiled.
       
3 July 1901, Severance (KS) News, pg. 4, col. 1:
TWO JASSACKS.
Sunday’s Ball-Game Spoiled By Babyish Fools and a Stick of an Umpire.
 
Chronicling Illinois
Google Books
Collected Works of Abraham Lincoln
Edited by Roy Prentice Basler
New Brunswick, NJ: Rutgers University Press
1953
Pg. 420:
Bass-Ackwards
He said he was riding bass-ackwards on a jass-ack, through a patton-ootch, on a pair of baddle-sags, stuffed full of binger-gred, when the animal steered at a scump, and the lirrup-steather broke, and throwed him in the forner of the kence and broke his pishing-fole.
 
Urban Dictionary
jassack
mfrndrmjr.
What the hell you doing over there you jassack!?!
#jackass#numbnuts#tool#moron#dipshit
by fletchfflecth April 14, 2014
   
Twitter
Easy Pronounce
@EasyPronounce18
How to Pronounce Jassack ↺RT❤ http://www.easypronounce.com/2548034  #newb #scrub #revoke #putkiboshon #prohibit #freeze #extinguish #twat
3:04 PM - 9 Jan 2018
       
Twitter
Ben Zimmer
@bgzimmer
Addendum: Lincoln is now credited by the @OED as the first known user of “bass-ackwards,” in a spooneristic letter he wrote in the 1840s. “He said he was riding bass-ackwards on a jass-ack through a patton-cotch…” http://languagelog.ldc.upenn.edu/nll/?p=36421
1:27 PM - 12 Feb 2019
   
Twitter
4 The Serious Seeker
@serious_seeker
Simple common sense, @POTUS. No indictment because you are president is NOT the same as “No Collusion.” STOP TELLING LIES, JASSACK TRUMP!
7:54 PM - 1 Jun 2019