1. Short form of Katherine and other names beginning with K.
2. From the Welsh name Cai or Cei, possibly a form of the Roman name Gaius. Sir Kay was one of the Knights of the Round Table in Arthurian legend. He first appears in Welsh tales as a brave companion of Arthur. In later medieval tales, notably those by the 12th-century French poet Chrétien de Troyes, he is portrayed as an unrefined boor.
3. Variant of Kai.