- monster (n.) early 14c., "malformed animal or human, creature afflicted with a birth defect," from O.Fr. monstre, mostre "monster, monstrosity" (12c.), and directly from L. monstrum "divine omen, portent, sign; abnormal shape; monster, monstrosity," figuratively "repulsive character, object of dread, awful deed, abomination," from root of monere "warn" (see MONITOR (Cf. monitor) (n.)). Abnormal or prodigious animals were regarded as signs or omens of impending evil. Extended by late 14c. to imaginary animals composed of parts of creatures (centaur, griffin, etc.). Meaning "animal of vast size" is from 1520s; sense of "person of inhuman cruelty or wickedness" is from 1550s. As an adjective, "of extraordinary size," from 1837. In Old English, the monster Grendel was an aglæca, a word related to aglæc "calamity, terror, distress, oppression."
Etymology dictionary. 2014.