means A creature of frightening, often unnatural form — or, by extension, a person of monstrous cruelty or a thing of monstrous size.

from From Latin 'monstrum,' a divine omen or portent — literally something shown or revealed, from 'monere,' to warn (the same root that gives us 'monitor' and 'admonish'). To the Romans a monster wasn't just terrifying; it was a sign from the gods that something had gone wrong. The word came into English through Old French 'monstre,' and over time the warning faded while the teeth remained.