The Collaborative International Dictionary of English v.0.48:

Disastrous \Dis*as"trous\, a. [Cf. F. d['e]sastreux. See
   Disaster.]
   1. Full of unpropitious stellar influences; unpropitious;
      ill-boding. [Obs.]
      [1913 Webster]

            The moon
            In dim eclipse, disastrous twilight sheds. --Milton.
      [1913 Webster]

   2. Attended with suffering or disaster; very unfortunate;
      calamitous; ill-fated; ending in utter failure or ruin;
      as, a disastrous day; a disastrous termination of an
      undertaking.
      [1913 Webster]

            Wherein I spake of most disastrous chances. --Shak.
      -- Dis*as"trous*ly, adv. -- Dis*as"trous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

disastrously
    adv 1: in a disastrous manner; "the real value of the trust
           capital may be disastrously less than when the trust
           began"