The Collaborative International Dictionary of English v.0.48:

Frightful \Fright"ful\, a.
   1. Full of fright; affrighted; frightened. [Obs.]
      [1913 Webster]

            See how the frightful herds run from the wood. --W.
                                                  Browne.
      [1913 Webster]

   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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

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,
           frightful, horrifying, horrible, ugly]
    2: extreme in degree or extent or amount or impact; "in a
       frightful hurry"; "spent a frightful amount of money" [syn:
       frightful, terrible, awful, tremendous]
    3: extremely distressing; "fearful slum conditions"; "a
       frightful mistake" [syn: fearful, frightful]