Synonyms for Monstrous:

adj.

atrocious (adjective)
wicked, bad, shocking, nefarious.
colossal (adjective)
cyclopean, jumbo, mountainous.
corrupt (adjective)
baneful.
cruel (adjective)
hateful, sinful, hard, unnatural, unkind.
detestable (adjective)
offensive, repugnant, revolting.
different (adjective)
unusual.
evil (adjective)
calamitous, sinful, reprehensible, repugnant, disgraceful, bad, destructive, terrible, atrocious, sinister, hideous, evil, loathsome, nefarious, repellent, damnable, detrimental, wicked, offensive, heinous, repulsive, foul, baneful, revolting, abominable.
offensive (adjective)
outrageous.

adv.

beastly (adverb)
repulsive, abominable.

n.

hard, enormous, uncharitable, hateful, unkind, cruel, pitiless, prodigious, stupendous, inhuman. right, plain, scandalous, grim, outrageous, shocking. heroic, mammoth, titanic, immense, elephantine, colossal, Pythonic, walloping, jumbo, vast, Bunyanesque, massive, gargantuan, monster, cyclopean, gigantic, mountainous, behemoth, Brobdingnagian, mastodonic, herculean, Massy, giant, huge, mighty, tremendous, whopping, Gigantesque, monumental. freakish, freaky, usual. monstrous (noun)
flagitious, heinous, grievous, evil, atrocious, wicked, large, ugly, unnatural, grotesque, big.
shocking (noun)
outrageous.
unnatural (noun)
abnormal, uncanny, unusual.

Other synonyms:

freaky, Gigantesque, walloping, inhuman, hateful, pitiless, Brobdingnagian, prodigious, elephantine, Massy, unkind, enormous, mastodonic, uncharitable, Pythonic, Bunyanesque, cyclopean. gigantic, monumental, scandalous, colossal, immense, titanic, whopping, gargantuan, vast, hard, tremendous, mountainous, shocking, huge. stupendous, freakish, heroic, herculean. mighty, massive. grim, monster, plain. vicious
cruel.

Usage examples for monstrous

  1. We should accept the evidence that some monstrous herd, making its way through a mountain pass, somehow went crazy and bolted for the plains. – This World Is Taboo by Murray Leinster