The kilt was a traditional Human garment, typically worn by Human males of Scottish descent. It consisted of a length of tartan fabric (the tartan pattern often corresponding to the wearer's clan or regimental allegiance) with sharp pleats in the back and flat aprons on the front; the kilt was worn with various dress accoutrements including a sporran (purse) in the front, shoulder plaid attached with brooch, hose and a wide kilt belt.
The idea for a kilt to be included as part of Scott's dress uniform was thought up by actor James Doohan. He later recalled, "They [the producers] immediately grabbed onto the idea. They sent to Scotland for the Scott tartan." (The World of Star Trek, 3rd ed., p. 94)