a.
1.
Frightful, terrible, terrific, horrible, alarming, portentous, formidable, appalling, dire, horrifying, horrific, harrowing, dreadful, fearful, awful, hideous.
2.
( Colloq. ) Shocking, offensive, disgusting, revolting, repulsive, odious, disagreeable, unpleasant, vile, abominable.