WordWeb Online

1. Causing or able to cause nausea
   "a loathsome smell";
   - nauseating, nauseous, noisome, queasy, offensive, sickening, vile, vomitous [US], loathful [archaic]
    
2. Highly offensive; arousing aversion or disgust
   "a loathsome disease";
   - disgusting, disgustful [archaic], distasteful, foul, loathly, repellent, repellant, repelling, revolting, skanky [informal], wicked, yucky [informal], gross-out [informal], loathful [archaic], yukky [informal]

See also: gut-wrenching, offensive, unwholesome

Encyclopedia: Loathsome