Jayhawkers, term applied to free-state guerrilla fighters opposed to the proslavery "border ruffians" during the struggle over Kansas in the years prior to the Civil War. Later, during the war, it was the nickname of the Seventh Kansas Cavalry, commanded by Colonel Charles R. Jennison. The origin of the word is uncertain, but it is believed to signify a bird that worries its prey. Today Kansans are sometimes called Jayhawkers.
See S. Z. Starr, Jennison's Jayhawkers (1974).