“Jassack” (or “jass-ack”) is a metathesis of the word “jackass.” The word “jassack” has been cited in print since at least 1839.
   
“Jassack” possibly is the source of the words “jass” and “jazz” in the time period of 1912-1915. “The HERALD man is not the only one on this low ground of sorrow that enjoys the melodious sound of the jassack band. If he will just come over to the Horn and attend one concert of (Harry—ed.) Park’s Jassack Band he will soon learn that we have troubles of our own” was printed in the Houston (MO) Herald on March 12, 1914. “Last Thursday night lightning must have struck at (Harry—ed.) Parks’ Jassack band and we regret to say it missed the jassacks and hit a tree close to the barn” was printed in the Houston (MO) Herald on September 17, 1914. It appears that these 1914 “jassack band” cites describe a “jazz band,” but the term “jazz band” would not be cited in print until 1916.
   
“Jazzack” (jazz + jassack) is sometimes used. “These musicians were nicknamed the Jassacks Band, an inverse arrangement of the well-known denizen of southern states, the jackass. Jassacks soon was turned into Jazzacks and the ‘acks’ was finally lost leaving the delightful residue, ‘Jazz’” was printed in The Daily Alaskan (Skagway, AK) on September 24, 1918. “Indeed judging from the influence so called jazz seems to exercise in certain cases upon the human body either in the act of playing or dancing reminds one most forcibly of the mentality of the jazzacks, with apologies to the latter” was printed in the Pacific Coast Musical Review (San Francisco, CA) on August 9, 1919.
 
A “jackass band” (featuring the actual animal) during the Civil War was described in The National Tribune (Washington, DC) on March 17, 1904. “WHAT JASS MEANS The best definition of jass music is believed to have been given by an Atchison man. He says the term jass band is short for jackass band” is a joke that was printed in the Wichita (KS) Beacon on October 13, 1917. ” The name jazz came from what natives of Louisiana called ‘Jackass bands,’ a combination made up of saw-mill and country folks playing on home-made instruments. From jackass bands came jass and then jazz” was printed in the Reno (NV) Evening Gazette on October 23, 1959. However, from 1910-1920 (the formative years of “jazz”), it does not appear that any band called itself a “jackass band.”
   
         
Chronicling America
17 March 1904, The National Tribune (Washington, DC), pg. 6, col. 6:
A. J. Smith’s Jackass Band.
EDITOR NATIONAL TRIBUNE: On the Tupelo campaign, the day we ar-we (sic) had the most unique band in the Union Army. The boys captured a jackass and a very large bass viol. They strapped the viol on the jack with the neck of the violin running along on top of the jack’s neck. As the jack was being taken away from home he set up a large braying. Between the notes of his brays one of the boys, who walked by his side, would second the brays by drawing the bow across the viol. The music was ludicrous, but loud and very entertaining. You could hear the boys yell in laughter more than a mile away. Our regiment guarded the wagon train that day. You can tell about your brass bands and fife and drum corps, but they were not to be compared with this band. I do not know what became of it as I did not see it after that day. It was a great success on the march. I would like to have seen it tried on dress parade. If any one of the comrades knows of a band that beats this I would like to hear it from him.—M. C. THOMPSON. Co. C., 122d Ill., Milwaukee, Ore.
     
13 March 1913, Houston (MO) Herald, pg. 1, col. 2:
Dr. P. A. Herrington is neither building railroad nor getting our lights in operation just now, but he has gathered together a very musical jassack brigade which keeps the neighborhood from being dull.
 
5 March 1914, Houston (MO) Herald, “Editorial and Otherwise,” pg. 4, col. 2:
The song of that jassack in town may be melodious and harmonious, but that kind of music is not enjoyed by the neighbors and might be classed as a public nuisance.
 
12 March 1914, Houston (MO) Herald, “Hartshorn” by Tip Top, pg. 7, col. 5:
The HERALD man is not the only one on this low ground of sorrow that enjoys the melodious sound of the jassack band. If he will just come over to the Horn and attend one concert of (Harry—ed.) Park’s Jassack Band he will soon learn that we have troubles of our own.
 
16 April 1914, Houston (MO) Herald, “Hartshorn” by Tip Top, pg. 3, col. 1:
Hartshorn is blest with a jassack band too; they have brayed till they are so weak they can’t finish the bray without resting, so they just help each other out. One will commence and bray till his breath gives out and then the other one will take it up and finish, which takes about an hour and a half. 
 
17 September 1914, Houston (MO) Herald, “Hartshorn” by Tip Top, pg. 3, col. 2:
Last Thursday night lightning must have struck at (Harry—ed.) Parks’ Jassack band and we regret to say it missed the jassacks and hit a tree close to the barn.
 
6 April 1916, Houston (MO) Herald, “Hartshorn” by Tip Top, pg. 8, col. 2:
Say, Mr. Editor, we have organized our Jassack band and practice every three minutes. How is your band this spring?
 
27 September 1917, The Weekly Kansas Chief (Troy, KS), pg. 2, col. 2:
In speaking of the “jaz band” one ought to tell the truth and go the whole route and call it Jackass band, which it sounds like.
 
13 October 1917, Wichita (KS) Beacon, pg. 8, col. 4:
WHAT JASS MEANS
The best definition of jass music is believed to have been given by an Atchison man. He says the term jass band is short for jackass band.
 
Chronicling America
24 September 1918, The Daily Alaskan (Skagway, AK), pg. 2, col. 2:
ORIGIN OF THE WORD “JAZZ”
How many times have you been asked the meaning of the word jazz?
 
We have run down the etymological derivation of the famous word. Carry this story at the tip of your tongue; you will find it of interest to everyone. There was at one time a trio of colored musicians in the south, a singer, a banjo player and an expert hummer on an empty lard tin. These musicians were nicknamed the Jassacks Band, an inverse arrangement of the well-known denizen of southern states, the jackass. Jassacks soon was turned into Jazzacks and the “acks” was finally lost leaving the delightful residue, “Jazz.”
   
18 April 1919, Alma (KS) Enterprise, “Our Soldier Boys Letters: Met Them With Butcher Knife,” pg. 4, col. 4:
These people are so ancient that if an American Military Jass band came through town they would all run in the house and crawl under the bed. Great place this; haven’t seen any wooden shoes but they do have oxen and little jassacks.
   
20 July 1919, Orlando (FL) Morning Sentinel, pg. 10, col. 2 ad:
Whether the word “jazz” comes from that trio of vagrant negroes named the Jackass—Jazzacks or Jazz Band, which travelled around the South some twenty years ago, of from “Stale Bread’s Spasm Band,” a group of New Orleans news boys who did musical stunts about the same time, may never be known, but Jazz dance music to-day has America sitting up nights and England and France smashing all precedents of propriety to hear and dance to Jazz rhythms.
The Yowell-Duckworth Co.—ed.)
 
Google Books
9 August 1919, Pacific Coast Musical Review (San Francisco, CA), “Concerning the Significance of Jazz” by Alfred Metzger, pg. 3, col. 1:
Indeed judging from the influence so called jazz seems to exercise in certain cases upon the human body either in the act of playing or dancing reminds one most forcibly of the mentality of the jazzacks, with apologies to the latter.
 
12 October 1919, Fort Wayne (IN) Journal-Gazette, section 4, pg. 4, col.  8:
WHERE THE WORD
“JAZZ” ORIGINATED.
Most people are aware of the fact that “jazz” music originated in the south, but perhaps few know just how the name itself started. The Columbia Record gives the following explanation: There was once a trio of dusky musicians, one a banjo player, one a singer and the third a maker of melodies by means of an empty tin can. This unusual trio came to be called the Jassacks band, the name being the popular inversion of the jackass, the famous solo singer of the southern states. Soon the name, according to the proverbial love for inaccuracy, was changed to Jazzacks and by the usual method of abbreviation developed finally into just plain jazz.
 
19 March 1921, The Billboard (Cincinnati, OH), “Musical Musings” by O. A. Peterson, pg. 56, col. 4:
Having seen so many pictures of different present day orchestras, showing the various postures and make-ups of the different “jazz hounds” caused Spencer F. Williams to write from Philadelphia with the suggestion that “the term ‘jazzacks’ be applied to such players.”
 
7 September 1929, St. Louis (MO) Post-Dispatch, “Of Making Many Books: The New Spaeth Book” by John G. Neihardt, pg. 5, col. 1:
THEY STILL SING OF LOVE. By Sigmund Spaeth. (Liveright).
(...) (Col. 2—ed.)
In his chapter on jazz, Mr. Spaeth goes beyond the field of music and shows that “the term covers the whole territory of modern extravagance, absurdity and distortion of values,” from our murders and channel-swimming welcomings to our best selling literature. He is inclined to favor as an explanation of the origin of the term that it was originally “jass,” short for “jass-ack,” the metathesis of “jackass.”
     
Fulton NY Post Cards
18 March 1932, Times-Union (Albany, NY), “Albany Today” by De Witt Schuyler, pg. 3, col. 1:
Anent a recent reference in this column to jazz music, I am informed that the word “jazz” resulted from the twisting of the name of the original “Jackass” band down in Memphis, to “Jazzack’s” band. That band had three members, a banjoist, a performer on a tin can, and a vocalist.
   
24 September 1937, The Bergen Evening Record (Hackensack, NJ), “Music News and Views: Did You Say ‘Jazz’?” by Roger S. Vreeland, pg. 22, col. 4:
It may be interesting, in passing, to make a note as to the origin of the word “jazz.” The following story is current: In Memphis (remember “Memphis Blues”?), a dusky trio consisting or a singer, a banjo player and a performer on a tin can became locally known as the Jackass Band.
 
The name was jocularly transposed to Jassack’s or Jazzack’s Band; whence the abbreviation Jazz.
 
Google Books
How They Become Name Bands:
The modern technique of danceband maestro
By Paul Specht
New York, NY: Fine Arts Publications
1941
Pg. 98:
One writer actually claims that JAZZ was originally “jass”, short for “jass-ack”, the metathesis of “jack-ass”!
   
23 October 1959, Reno (NV) Evening Gazette, entertainment sec., pg. 6, col. 1:
MUSICIANS HALL OF FAME
SELECTS GUY LOMBARDO
AS FOUNDING MEMBER
Guy Lombardo has been elected to The Musicians Hall of Fame according to Gene Austin, sponsor of the project.
(...) (Col. 2—ed.)
“Nick La Rocca, who wrote Tiger Rag in 1916 is responsible for the jazz style of music. he introduced this before World War I in New Orleans with his Original Dixieland Jazz Band. The name jazz came from what natives of Louisiana called “Jackass bands,” a combination made up of saw-mill and country folks playing on home-made instruments. From jackass bands came jass and then jazz, Austin says.
 
12 July 1964, San Francisco (CA) Examiner, “Jazz: Gene Austin From Blues Country” by Richard Hadlock, Show Time sec., pg. 15, cols. 1-2:
‘Shocking’
“We called them ‘jackass’ bands when I was a kid in Louisiana,” he (Gene Austin—ed.) said. “I guess that’s because they were always imitating mules or horses or roosters on their instruments. These were little groups that played in the small towns where I was raised. They had combs with tissue paper, jugs, washboards and sometimes legitimate instruments.”
 
“They used to hang around the tenderloin sections, and I believe the people in the sporting houses gave them the name ‘jass’—a word that used to be considered pretty shocking. Anyway, the Victor Record company changed it to jazz when some wags insisted on rubbing off the “j” on their ads.