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®

disastrously
     adv : in a disastrous manner; "the real value of the trust capital
           may be disastrously less than when the trust began"