n.1 an imaginary creature, usu. large and frightening, compounded of incongruous elements.
2 an inhumanly cruel or wicked person.
3 a misshapen animal or plant.
4 a large hideous animal or thing (e.g. a building).
5 (attrib.) huge; extremely large of its kind.
Etymology: ME f. OF monstre f. L monstrum portent, monster f. monere warn