Reposted from the Scotland Forum From "The Surnames of Scotland" by Dr. G.F. Black Keay is a variant of Kay. There was an old family named Kay in Yorkshire. Kay and Kaye occur in various parts of England as far back as 13th century. The Welsh forename Kei comes from the Latin Caius.
It may be connected to the MacKay or Macaoidh family. The MacKays of the Isle of Man dropped the Mac. Early examples are Kee 1610, Kay 1617, Key 1616, Kie 1618. Some Scottish branches may have dropped the Mac earlier. The spelling Keay was common in Perthshire.
Other spellings: Qua 1317, Ka 1399, Kaa 1370, Cay 1474, Kay 1552, Kae 1667.