WordWeb Online

1. Abnormally large
    
2. Shockingly brutal or cruel
   "no excess was too monstrous for them to commit"
   - atrocious, flagitious, grievous
    
3. Distorted and unnatural in shape or size; abnormal and hideous
   "twisted into monstrous shapes"
   - grotesque

See also: big, evil, large, monster, monstrosity, monstrously, ugly

Encyclopedia: Monstrous