The origin of the expression is found in the Mediterranean beach Spanish during the middle ages, where a series of watchtowers was created to monitor the arrival of Arab Pirate ships from the coast. Spotted on the horizon the Moors boats the lookout shouted " there are Moors on the coast! " and they lit bonfires that along with the Bell warned neighbouring populations to confront the invaders. Last time the sentence took the meaning of warning to take care of someone close and indiscreet, or some superior who hide.
