Source : Webster's Revised Unabridged Dictionary (1913)

Frightful \Fright"ful\, a.
   1. Full of fright; affrighted; frightened. [Obs.]

            See how the frightful herds run from the wood. --W.
                                                  Browne.

   2. Full of that which causes fright; exciting alarm;
      impressing terror; shocking; as, a frightful chasm, or
      tempest; a frightful appearance.

   Syn: Terrible; dreadful; alarming; fearful; terrific; awful;
        horrid; horrible; shocking.

   Usage: {Frightful}, {Dreadful}, {Awful}. These words all
          express fear. In frightful, it is a sudden emotion; in
          dreadful, it is deeper and more prolonged; in awful,
          the fear is mingled with the emotion of awe, which
          subdues us before the presence of some invisible
          power. An accident may be frightful; the approach of
          death is dreadful to most men; the convulsions of the
          earthquake are awful.

Source : WordNet®

frightful
     adj 1: provoking horror; "an atrocious automobile accident"; "a
            frightful crime of decapitation"; "an alarming, even
            horrifying, picture"; "war is beyond all words
            horrible"- Winston Churchill; "an ugly wound" [syn: {atrocious},
             {horrifying}, {horrible}, {ugly}]
     2: extreme in degree; "in a frightful hurry"; "spent a
        frightful amount of money"
     3: extremely distressing; "fearful slum conditions"; "a
        frightful mistake"; "suffered terrible thirst" [syn: {fearful},
         {terrible}]