Source : Webster's Revised Unabridged Dictionary (1913)

Disastrous \Dis*as"trous\, a. [Cf. F. d['e]sastreux. See
   {Disaster}.]
   1. Full of unpropitious stellar influences; unpropitious;
      ill-boding. [Obs.]

            The moon In dim eclipse, disastrous twilight sheds.
                                                  --Milton.

   2. Attended with suffering or disaster; very unfortunate;
      calamitous; ill-fated; as, a disastrous day; a disastrous
      termination of an undertaking.

            Wherein I spake of most disastrous chances. --Shak.
      -- {Dis*as"trous*ly}, adv. -- {Dis*as"trous*ness}, n.

Source : WordNet®

disastrous
     adj : (of events) having extremely unfortunate or dire
           consequences; bringing ruin; "the stock market crashed
           on Black Friday"; "a calamitous defeat"; "the battle
           was a disastrous end to a disastrous campaign"; "such
           doctrines, if true, would be absolutely fatal to my
           theory"- Charles Darwin; "it is fatal to enter any war
           without the will to win it"- Douglas MacArthur; "a
           fateful error" [syn: {black}, {calamitous}, {fatal}, {fateful}]