The Collaborative International Dictionary of English v.0.48:

Ruinous \Ru"in*ous\, a. [L. ruinosus: cf. F. ruineux. See
   Ruin.]
   1. Causing, or tending to cause, ruin; destructive; baneful;
      pernicious; as, a ruinous project.
      [1913 Webster]

            After a night of storm so ruinous.    --Milton.
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   2. Characterized by ruin; ruined; dilapidated; as, an
      edifice, bridge, or wall in a ruinous state.
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   3. Composed of, or consisting in, ruins.
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            Behold, Damascus . . . shall be a ruinous heap.
                                                  --Isa. xvii.
                                                  1.
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   Syn: Dilapidated; decayed; demolished; pernicious;
        destructive; baneful; wasteful; mischievous.
        [1913 Webster] -- Ru"in*ous*ly, adv. --
        Ru"in*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

ruinously
    adv 1: in a ruinous manner or to a ruinous degree; "ruinously
           high wages"