Child, #200 misssed the boat on this one. Probably made up in 1643 to embarrass the Earl of Cassillis, a staunch Covenanter. His (loving) wife has died the previous December. The tune was already known as "Lady Cassillis' Lilt" at the time (Skene MS). Child though 'Cassillis' was a corruption of 'castle', but it's the other way around.