The military order Havoc! was a signal given to the English military forces in the Middle Ages to direct the soldiery (in Shakespeare’s parlance ‘the dogs of war’) to pillage and chaos.

The ‘let slip’ is an allusion to the slip collars that were used to restrain dogs and were easily ‘let slip’ to allow the dogs to run and hunt.

The Black Book of the Admiralty, 1385 is a collection of laws in French and Latin that relate to the organisation of the English Navy. In the ‘Ordinances of War of Richard II’ in that book we find:

“Item, qe nul soit si hardy de crier havok.”
[Item: No one should be so foolish as to cry havoc.]