WordWeb Online

1. Causing fear, dread or terror
   "a dreadful storm";
   - awful, dire, direful, dread, dreaded, fearful, fearsome, frightening, horrendous, horrific, terrible
    
2. Exceptionally bad or displeasing
   "dreadful manners";
   - atrocious, abominable, awful, painful, terrible, unspeakable, horrible
    
3. Extremely disagreeable and unpleasant
   "don't go out, the weather is dreadful"

See also: alarming, bad, disagreeable