early 15c., from Anglo-French havok in phrase crier havok "cry havoc" (late 14c.), a signal to soldiers to seize plunder, from Old French havot "pillaging, looting," related to haver "to seize, grasp," hef "hook," probably from a Germanic source (see hawk (n.)), or from Latin habere "to have, possess." General sense of "devastation" first recorded late 15c.
Sound an alarm or warning, as in In his sermon the pastor cried havoc to the congregation's biases against gays. The noun havoc was once a command for invaders to begin looting and killing the defenders' town. Shakespeare so used it in Julius Caesar (3:1): “Cry 'Havoc' and let slip the dogs of war.” By the 19th century the phrase had acquired its present meaning.