Colly my Cow
A corruption of Calainos, the most ancient of Spanish ballads. Calainos the Moor asked a damsel to wife, who said the price of winning her should be the heads of the three paladins of Charlemagne, named Rinaldo, Roland, and Olivier. Calainos went to Paris and challenged the paladins. First Sir Baldwin, the youngest knight, accepted the challenge and was overthrown; then his uncle Roland went against the Moor and smote him.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894