The Collaborative International Dictionary of English v.0.48:

Monstrous \Mon"strous\, adv.
   Exceedingly; very; very much. "A monstrous thick oil on the
   top." --Bacon.
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         And will be monstrous witty on the poor. --Dryden.
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The Collaborative International Dictionary of English v.0.48:

Monstrous \Mon"strous\ (m[o^]n"str[u^]s), a. [OE. monstruous, F.
   monstrueux, fr. L. monstruosus, fr. monstrum. See Monster.]
   1. Marvelous; strange. [Obs.]
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   2. Having the qualities of a monster; deviating greatly from
      the natural form or character; abnormal; as, a monstrous
      birth. --Locke.
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            He, therefore, that refuses to do good to them whom
            he is bound to love . . . is unnatural and monstrous
            in his affections.                    --Jer. Taylor.
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   3. Extraordinary in a way to excite wonder, dislike,
      apprehension, etc.; -- said of size, appearance, color,
      sound, etc.; as, a monstrous height; a monstrous ox; a
      monstrous story.
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   4. Extraordinary on account of ugliness, viciousness, or
      wickedness; hateful; horrible; dreadful.
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            So bad a death argues a monstrous life. --Shak.
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   5. Abounding in monsters. [R.]
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            Where thou, perhaps, under the whelming tide
            Visitest the bottom of the monstrous world.
                                                  --Milton.
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WordNet (r) 3.0 (2006):

monstrous
    adj 1: abnormally large
    2: shockingly brutal or cruel; "murder is an atrocious crime";
       "a grievous offense against morality"; "a grievous crime";
       "no excess was too monstrous for them to commit" [syn:
       atrocious, flagitious, grievous, monstrous]
    3: distorted and unnatural in shape or size; abnormal and
       hideous; "tales of grotesque serpents eight fathoms long that
       churned the seas"; "twisted into monstrous shapes" [syn:
       grotesque, monstrous]

Moby Thesaurus II by Grady Ward, 1.0:

311 Moby Thesaurus words for "monstrous":
   Atlantean, Brobdingnagian, Cyclopean, Gargantuan, Herculean,
   Homeric, a bit much, abandoned, abject, abominable, absurd,
   abysmal, amplitudinous, appalling, arrant, astronomic,
   astronomical, atrocious, awesome, awful, awfully, bad, bandy,
   bandy-legged, barbaric, barbarous, baroque, base, beastly,
   beggarly, beneath contempt, beyond belief, big, bizarre, black,
   blamable, blameworthy, blemished, bloated, boundless, bowlegged,
   brutal, brutish, bulky, cacogenic, cheesy, club-footed, cockamamie,
   colossal, contemptible, cosmic, cracking, crazy, criminal, cruel,
   crummy, crying, damnable, dark, debased, defaced, deformed,
   degraded, deplorable, depraved, desperate, despicable, detestable,
   dire, dirty, disfigured, disgraceful, disgusting, dreadful,
   dwarfed, egregious, elephantine, enormous, epic, evil, exaggerated,
   excessive, execrable, exorbitant, extensive, extravagant, extreme,
   extremely, fabulous, fancy, fantastic, fateful, fetid, fiendish,
   filthy, flagitious, flagrant, flatfooted, foolish, foul, freak,
   freakish, frightful, fulsome, galactic, gargantuan, gargoylish,
   ghoulish, giant, giantlike, gigantic, glaring, gluttonous,
   grandiose, grave, grievous, grisly, gross, grotesque, gruesome,
   hateful, heinous, hellish, heroic, hideous, high, high-flown,
   horrendous, horrible, horrid, horrific, horrifying, huge,
   hyperbolic, hypertrophied, ill-made, ill-proportioned, ill-shaped,
   immeasurable, immense, immoderate, impressive, improper,
   incontinent, incredible, infamous, infinite, inhuman, iniquitous,
   inordinate, insensitive, intemperate, jumbo, king-size, knavish,
   knock-kneed, lamentable, large, laughable, little, loathsome,
   lousy, low, low-down, ludicrous, lumpen, magnificent, malformed,
   mammoth, mangy, marred, massive, massy, mean, measly, merciless,
   mightily, mighty, misbegotten, miserable, misproportioned,
   misshapen, monster, monumental, mortal, mountainous, mutilated,
   nasty, naughty, nauseating, nauseous, nefarious, noisome,
   nonsensical, notorious, obnoxious, odious, offensive, ominous,
   out of bounds, out of shape, out of sight, outrageous, outre,
   outsize, overbig, overdeveloped, overgreat, overgrown, overlarge,
   overmuch, overweening, paltry, peccant, petty, pigeon-toed,
   pitiable, pitiful, poky, poor, poppycockish, portentous,
   preposterous, prodigious, profound, pug-nosed, rachitic, rank,
   rattling, regrettable, repellent, reprehensible, reprobate,
   reptilian, repulsive, revolting, rickety, ridiculous, rococo,
   rotten, ruthless, sad, savage, scabby, scandalous, schlock,
   scrubby, scruffy, scummy, scurvy, shabby, shameful, shameless,
   shapeless, shocking, shoddy, showy, simous, sinful, sizable, small,
   snapping, snub-nosed, sordid, spacious, splendid, squalid, steep,
   stiff, stumpy, stupendous, superb, swaybacked, talipedic, teratic,
   teratogenic, teratoid, terrible, titanic, too bad, too much,
   towering, tremendous, truncated, ugly, unbridled, unclean,
   unconscionable, undue, unforgivable, unmentionable, unpardonable,
   unreasonable, unrestrained, unshapely, unspeakable, unworthy, vast,
   vicious, vile, villainous, voluminous, weighty, weird, wicked,
   wild, woeful, worst, worthless, wretched, wrong