E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To cry out haro to anyone. To denounce his misdeeds, to follow him with hue and cry. Ha rou was the ancient Norman hue-and-cry, and the exclamation made by those who wanted assistance, their person or property being in danger. It is similar to our cry of Police! Probably our halloo is the same word.
In the Channel Isles, Ha! ho! à laide, mon prince! is a protest still in vogue when ones property is endangered, or at least was so when I lived in Jersey. It is supposed to be an appeal to Rollo, king of Normandy, to come to the aid of him suffering wrongfully.