adj. 1. such as to cause fright; dreadful, terrible, or alarming: a frightful explosion. 2. horrible, shocking, or revolting: The storm did frightful damage. 3. unpleasant; disagreeable: We had a frightful time. 4. very great; extreme: That actor is a frightful ham.[1200–50]fright′ful•ly,adv. fright′ful•ness,n. ThesaurusSeefrightful