E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A military cry to general massacre without quarter. This cry was forbidden in the ninth year of Richard II. on pain of death. Probably it was originally used in hunting wild beasts, such as wolves, lions, etc., that fell on sheepfolds, and Shakespeare favours this suggestion in his Julius Csar, where he says Até shall cry havock! and let slip the dogs of war. (Welsh, hafog, devastation; Irish, arvach; compare Anglo-Saxon havoc, a hawk.)