The origin of the term "Jayhawker" is uncertain. The term was adopted as a nickname by a group of emigrants traveling to California in 1849.  The origin of the term may go back as far as the Revolutionary War, when it was reportedly used to describe a group associated with American patriot John Jay.

The term became part of the lexicon of the Missouri-Kansas border in about 1858, during the Kansas territorial period. The term was used to describe militant bands nominally associated with the free-state cause. One early Kansas history contained this succinct characterization of the jayhawkers:

“Confederated at first for defense against pro-slavery outrages, but ultimately falling more or less completely into the vocation of robbers and assassins, they have received the name --- whatever its origin may be -- of jayhawkers.”

Another historian of the territorial period described the jayhawkers as bands of men that were willing to fight, kill, and rob for a variety of motives that included defense against pro-slavery "Border Ruffians", abolition, driving pro-slavery settlers from their claims of land, revenge, and/or plunder and personal profit.

While the “Bleeding Kansas” era is generally regarded as beginning in 1856, the earliest documented uses of the term "jayhawker" during the Kansas troubles were in late 1858, after the issue of slavery in Kansas had essentially been decided in favor of the Free State cause. The most credible explanation of why the term (as applied to the Kansas troubles) emerged at that time is provided in the retrospective account of Kansas newspaperman John McReynolds.
McReynolds reportedly picked up the term from Pat Devlin, a Free State partisan described as "nothing more nor less than a dangerous bully."

In mid-1858, McReynolds asked Devlin where he had acquired two fine horses that he had recently brought into the town of Osawatomie. Devlin replied that he "got them as the Jayhawk gets its birds in Ireland,” which he explained as follows:

“In Ireland a bird, which is called the Jayhawk, flies about after dark, seeking the roosts and nests of smaller birds, and not only robs nests of eggs, but frequently kills the birds.”

McReynolds understood Devlin had acquired his horses in the same manner the Jayhawk got its prey, and used the term in a Southern Kansas Herald newspaper column to describe a case of theft in the ongoing partisan violence. The term was quickly picked up by other newspapers, and "Jayhawkers" soon came to denote the militants and thieves affiliated with the Free State cause.

The meaning of the jayhawker term evolved in the opening year of the Civil War. When Charles Jennison, one of the territorial-era jayhawkers, was authorized to raise a regiment of cavalry to serve in the Union army, he characterized the unit as the "Independent Kansas Jay-Hawkers" on a recruiting poster.

The regiment was officially termed the 7th Regiment Kansas Volunteer Cavalry, but was popularly known as Jennison's Jayhawkers. Thus, the term became associated with Union troops from Kansas. After the regiment was banished from the Missouri-Kansas border in the spring of 1862, it went on to participate in several battles including Union victories of the Battle of Iuka and the Second Battle of Corinth.

Late in the war, the regiment returned to Kansas and contributed to Union victory in one of the last major battles in the Missouri-Kansas theatre, the Battle of Mine Creek.

The jayhawker term was applied not only to Jennison and his command, but to any Kansas troops engaged in predatory operations against the civilian population of western Missouri, in which the plundering and arson that characterized the territorial struggles were repeated, but on a much larger scale.

For example, the term "jayhawkers" also encompassed Senator Jim Lane and his Kansas Brigade, which sacked and burned Osceola, Missouri in the opening months of the war after their defeat by Sterling Price's Missouri State Guard in the Battle of Dry Wood Creek.  

A number of other smaller Missouri towns, and large swaths of the Missouri countryside, were similarly plundered and laid waste by Union forces as back and forth raids were undertaken by forces from both sides.

However, lending to confusion over the precise meaning of the jayhawker term along the Missouri-Kansas border, it also continued to be used to describe outright criminals like Marshall Cleveland that operated on a smaller scale and outside the Union military command, but still under the cover of supposed Unionism.  A newspaper reporter traveling through Kansas in 1863 provided definitions of jayhawker and associated terms:

Jayhawkers, Red Legs, and Bushwhackers are everyday terms in Kansas and Western Missouri. A Jayhawker is a Unionist who professes to rob, burn out and murder only rebels in arms against the government. A Red Leg is a Jayhawker originally distinguished by the uniform of red leggings. A Red Leg, however, is regarded as more purely an indiscriminate thief and murderer than the Jayhawker or Bushwhacker. A Bushwhacker is a rebel Jayhawker, or a rebel who bands with others for the purpose of preying upon the lives and property of Union citizens. They are all lawless and indiscriminate in their iniquities.

The depredations of the jayhawkers contributed to the descent of the Missouri-Kansas border region into some of the most vicious guerrilla fighting of the Civil War. In the first year of the war, much of the movable wealth in western Missouri had been transferred to Kansas, and large swaths of western Missouri had been laid waste, by an assortment of Kansas jayhawkers ranging from outlaws and independent military bands to rogue federal troops such as Lane's Brigade and Jennison's Jayhawkers

 In February 1862, the Union command instituted martial law due to "the crime of armed depradations or jay-hawking having reached a height dangerous to the peace and posterity to the whole State (Kansas) and seriously compromising the Union cause in the border counties of Missouri."  

One expert on the jayhawkers stated that the Border War would have been bad enough given the fighting between secessionist and unionist Missourians, "but it was basically Kansas craving for revenge and Kansas craving for loot that set the tone of the war.

Nowhere else, with the grim exception of the East Kentucky and East Tennessee mountains, did the Civil War degenerate so completely into a squalid, murderous, slugging match as it did in Kansas and Missouri."

] The most infamous event in this war of raids and reprisals was Confederate leader William Quantrill's retaliatory attack on Lawrence, Kansas known as the Lawrence Massacre.

In response to Quantrills' raid, the Union command issued Order No. 11, the forced depopulation of specified Missouri border lands. Intended to eliminate sanctuary and sustenance for pro-Confederate guerilla fighters, it was enforced by troops from Kansas, and provided an excuse for a final round of pludering, arson, and summary execution perpretated against the civilian population of western Missouri.  In the words of one observer, "the Kansas-Missouri border was a disgrace even to barbarism."

As the war continued, the "jayhawker" term came to be used by Confederates as a derogatory term for any troops from Kansas, but the term also had different meanings in different parts of the county.

In Arkansas, the term was used by Confederate Arkansans as an epithet for any marauder, robber, or thief (regardless of Union or Confederate affiliation). In Louisiana, the term was used to describe anti-Confederate guerrillas, as well as free-booting bands of draft dodgers and deserters.

Over time, proud of their State's contributions to the end of slavery and the preservation of the Union, Kansans embraced the "Jayhawker" term. The term came to be applied to people or items related to Kansas, similar to the terms "Hoosier" for Indiana, "Sooner" for Oklahoma, "Tar Heel" for North Carolina, and "Buckeye" for Ohio.

[edit]https://en.wikipedia.org/wiki/Jayhawker