The Collaborative International Dictionary of English v.0.48:

Monster \Mon"ster\, a.
   1. Monstrous in size. --Pope.
      [1913 Webster]

   2. Enormous or very powerful; as, he drove a monster Harley.
      [informal]
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Monster \Mon"ster\ (m[o^]n*st[~e]r), 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.
      [1913 Webster]

            A monster or marvel.                  --Chaucer.
      [1913 Webster]

   2. Specifically, an animal or plant departing greatly from
      the usual type, as by having too many limbs.
      [1913 Webster]

   3. Any thing or person of unnatural or excessive ugliness,
      deformity, wickedness, or cruelty.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Monster \Mon"ster\, v. t.
   To make monstrous. [Obs.] --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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, monster, colossus]
    3: a person or animal that is markedly unusual or deformed [syn:
       freak, monster, monstrosity, lusus naturae]
    4: a cruel wicked and inhuman person [syn: monster, fiend,
       devil, demon, ogre]
    5: (medicine) a grossly malformed and usually nonviable fetus
       [syn: monster, teras]