Source : Webster's Revised Unabridged Dictionary (1913)

Monster \Mon"ster\, n. [OE. monstre, F. monstre, fr. L.
   monstrum, orig., a divine omen, indicating misfortune; akin
   of monstrare to show, point out, indicate, and monere to
   warn. See {Monition}, and cf. {Demonstrate}, {Muster}.]
   1. Something of unnatural size, shape, or quality; a prodigy;
      an enormity; a marvel.

            A monster or marvel.                  --Chaucer.

   2. Specifically, an animal or plant departing greatly from
      the usual type, as by having too many limbs.

   3. Any thing or person of unnatural or excessive ugliness,
      deformity, wickedness, or cruelty.

Monster \Mon"ster\, a.
   Monstrous in size. --Pope.

Monster \Mon"ster\, v. t.
   To make monstrous. [Obs.] --Shak.

Source : WordNet®

monster
     n 1: an imaginary creature usually having various human and
          animal parts
     2: someone or something that is abnormally large and powerful
        [syn: {giant}, {goliath}, {behemoth}, {colossus}]
     3: a person or animal that is markedly unusual or deformed
        [syn: {freak}, {monstrosity}, {lusus naturae}]
     4: a cruel wicked and inhuman person [syn: {fiend}, {devil}, {demon},
         {ogre}]
     5: (medicine) a grossly malformed and usually nonviable fetus
        [syn: {teras}]