A couple weeks ago as Halloween stuff was everywhere, my family was talking one night about the word monster and where it might have come from. We were musing that since it ends in -ster, and -ster usually indicates “someone who does a particular task” (like spinster is one who spins, mobster is one who belongs to a mob, hipster is one who is hip), then clearly monster is someone who…er…mons. Whatever that might be. 😉 We decided it was someone who moans and proceeded to be moaning monsters…
Turns out this time we were WAY off, ha ha. In this case, that -ster ending is coincidental. Monster in fact comes from the Latin monstrum, which means “a bad omen, portend, or sign; something misshapen.”
See, in the ancient days, any time an animal or person was born with some deformity, it was believed that it was a sign of bad things to come. And in fact, after a bad thing happened, historians would search for the bad omen that should have warned them about it. The first chunk of Histories by Herodotus if full of all the evil portends, monsters, and abnormalities that had been found in the country in the decades leading up to the war. So a creature–whether human or animal–who displayed these visible signs would be called monstrous or a monster.
If you look at the root of that word itself, it’s actually from monere, “to remind or bring to mind.” Which is what an omen does, I suppose.