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]

Moby Thesaurus II by Grady Ward, 1.0:

205 Moby Thesaurus words for "monster":
   Argus, Atlantean, Briareus, Brobdingnagian, Cerberus, Charybdis,
   Cyclopean, Cyclops, Dracula, Echidna, Frankenstein, Gargantuan,
   Gorgon, Harpy, Herculean, Homeric, Hydra, Loch Ness monster,
   Mafioso, Medusa, Minotaur, Pegasus, Python, Scylla, Sphinx, Talos,
   Typhon, Wolf-man, Young Turk, abortion, abysmal, amplitudinous,
   ape-man, astronomic, astronomical, awesome, baboon, bag, beast,
   beldam, berserk, berserker, blemish, blot, bogey, bogeyman, bomber,
   boundless, brute, bugaboo, bugbear, bulky, centaur, chimera,
   cockatrice, colossal, cosmic, deformity, demon, devil,
   devil incarnate, dinosaur, dog, dragon, drake, elephant,
   elephantine, enormous, epic, extensive, eyesore, fee-faw-fum,
   fiend, fiend from hell, fire-eater, firebrand, freak,
   freak of nature, fright, frightener, fury, galactic, gargantuan,
   gargoyle, ghost, ghoul, giant, giantlike, gigantic, goon, gorilla,
   griffin, gunsel, hag, hardnose, harpy, harridan, hell-raiser,
   hellcat, hellhound, hellion, hellkite, heroic, hippo, hippocampus,
   hippopotamus, hobgoblin, holy terror, hood, hoodlum, horror,
   hothead, hotspur, huge, immeasurable, immense, incendiary, incubus,
   infinite, jumbo, killer, king-size, lamia, large, leviathan,
   mad dog, madcap, mammoth, massive, massy, mastodon, mermaid,
   merman, mess, mighty, miscreation, missing link, monstrosity,
   monstrous, monumental, mountainous, mugger, mutant, mutation,
   nightmare, nixie, no beauty, ogre, ogress, outsize, overgrown,
   phantom, prodigious, profound, rapist, revenant, revolutionary,
   roc, salamander, satyr, savage, scarebabe, scarecrow, scarer,
   sea horse, sea serpent, she-wolf, sight, siren, sizable, spacious,
   specter, spitfire, stupendous, succubus, teratism, termagant,
   terror, terrorist, tiger, tigress, titanic, tough, tough guy,
   towering, tremendous, troll, ugly customer, ugly duckling, unicorn,
   vampire, vast, violent, virago, vixen, voluminous, weighty,
   werewolf, whale, wild beast, windigo, witch, wolf, xiphopagus,
   zombie



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MONSTER, physiology, persons. An animal which has a conformation contrary to
the order of nature. Dunglison's Human Physiol. vol. 2, p. 422.
     2. A monster, although born of a woman in lawful wedlock, cannot
inherit. Those who have however the essential parts of the human form and
have merely some defect of coformation, are capable of inheriting, if
otherwise qualified. 2 Bl. Com. 246; 1 Beck's Med. Jurisp. 366; Co. Litt. 7,
8; Dig. lib. 1, t. 5, l. 14; 1 Swift's Syst. 331 Fred. Code, Pt. 1, b. 1, t.
4, s. 4.
     3. No living human birth, however much it may differ from human shape,
can be lawfully destroyed. Traill. Med. Jur. 47, see Briand, Med. Leg. 1ere
part. c. 6, art. 2, Sec. 3; 1 Fodere, Med. Leg. Sec. 402-405.