Source : Webster's Revised Unabridged Dictionary (1913)

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.

            After a night of storm so ruinous.    --Milton.

   2. Characterized by ruin; ruined; dilapidated; as, an
      edifice, bridge, or wall in a ruinous state.

   3. Composed of, or consisting in, ruins.

            Behold, Damascus . . . shall be a ruinous heap.
                                                  --Isa. xvii.
                                                  1.

   Syn: Dilapidated; decayed; demolished; pernicious;
        destructive; baneful; wasteful; mischievous. --
        {Ru"in*ous*ly}, adv. -- {Ru"in*ous*ness}, n.

Source : WordNet®

ruinous
     adj 1: extremely harmful; bringing physical or financial ruin; "a
            catastrophic depression"; "catastrophic illness"; "a
            ruinous course of action" [syn: {catastrophic}]
     2: causing injury or blight; especially affecting with sudden
        violence or plague or ruin; "the blasting effects of the
        intense cold on the budding fruit"; "the blasting force of
        the wind blowing sharp needles of sleet in our faces"; "a
        ruinous war" [syn: {blasting}]