The Oxford Thesaurus An A-Z Dictionary of Synonyms - monstrous adj.

1) awful, horrible, horrid, horrific, horrendous, horrifying, hideous, ugly, nightmarish, dreadful, heinous, grisly, gruesome, disgusting, nauseous, nauseating, repulsive, repellent, revolting, frightful, grotesque, hellish, ghoulish, freakish, fiendish, barbaric, barbarous, savage, inhuman, merciless, ruthless, brutal, brutish, beastly: How could anyone treat another human being with such monstrous cruelty! 2 outrageous, shocking, scandalous, atrocious, appalling, wicked, villainous, evil, vile, insensitive, cruel, base, debased, shameful, shameless, infamous, disgraceful, nefarious, egregious, foul, vicious, flagitious, loathsome, depraved: It is monstrous to suggest that money could compensate for the loss of a child. It was a monstrous trick to make him think he had failed the exam when he'd got the top mark. 3) gigantic, giant, huge, vast, enormous, colossal, monster, gargantuan, jumbo, immense, tremendous, titanic, prodigious, massive, towering, elephantine, mammoth: They are planning to build a monstrous shopping mall in the town centre.