Jay·hawk·er (jā'hô'kər) n.

Jayhawker: Guerrilla fighter from Kansas during the American Civil War, who often clashed with pro-slavery partisans (Bushwhackers) and Missouri militia units.

Organizers of the Jayhawkers included James H. Lane (R) and Charles "Doc" Jennison (pictured). Jennison's raids into Missouri were thorough leaving five counties in western Missouri wasted, save for the standing brick chimneys of the two-storey period houses, which are still called "Jennison Monuments" in the areas. Lane and his band of militants wore red gaiters, earning them the nickname "Redlegs", or "Redleggers". This moniker was often used interchangeably with the term "Jayhawkers" (although it was sometimes used to refer specifically to Jayhawkers who refused to join units officially sanctioned by the US Army). Lane's attack on Osceola, Missouri, inspired the Clint Eastwood film The Outlaw Josey Wales.

Within Kansas, the Jayhawkers were not always popular because, in the absence of federal support, they sometimes supplied themselves by stealing horses and supplies from farmers.

The Jayhawk is the official mascot of the University of Kansas.