define monstrous
frightful or hideous, especially in appearance; extremely ugly. 2. shocking or revolting; outrageous: monstrous cruelty. 3. extraordinarily great; huge; immense: a monstrous building.
Monster is derived from the Latin monstrum, meaning monster or portent, unnatural thing/event regarded as omen/sign/portent. The academic study of the literary and cultural heritage of monsters is known as monstrophy.