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.
      [1913 Webster]

   2. Characterized by ruin; ruined; dilapidated; as, an
      edifice, bridge, or wall in a ruinous state.
      [1913 Webster]

   3. Composed of, or consisting in, ruins.
      [1913 Webster]

            Behold, Damascus . . . shall be a ruinous heap.
                                                  --Isa. xvii.
                                                  1.
      [1913 Webster]

   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):

ruinous
    adj 1: extremely harmful; bringing physical or financial ruin;
           "a catastrophic depression"; "catastrophic illness"; "a
           ruinous course of action" [syn: catastrophic,
           ruinous]
    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, ruinous]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "ruinous":
   ablative, annihilative, ausgespielt, baleful, baneful, bankrupt,
   battered, beat-up, beaten up, bedraggled, biodegradable, black,
   blasted, blighted, blowzy, broken, broken-down, calamitous,
   careless, cataclysmal, cataclysmic, catastrophic, chintzy,
   consuming, consumptive, corrosive, crippling, crumbling, deadly,
   decomposable, decomposing, decrepit, degradable, deleterious,
   demolishing, demolitionary, depredatory, derelict, desolated,
   desolating, destroyed, destroying, destructive, devastated,
   devastating, dilapidated, dire, disastrous, disintegrable,
   disintegrated, disintegrating, disintegrative, disjunctive,
   disruptive, done for, done in, doomful, down-and-out,
   drabbletailed, draggled, draggletailed, dusty, erosive, fallen,
   fatal, fateful, finished, fratricidal, frowzy, frumpish, frumpy,
   fusty, gone to pot, gone to seed, grievous, grubby, harmful,
   in rags, in ruins, informal, injurious, internecine, irremediable,
   kaput, loose, lumpen, messy, mildewed, moldering, moldy,
   moss-grown, moth-eaten, mussy, musty, nasty, negligent, nihilist,
   nihilistic, noxious, overthrown, pernicious, poisonous, poky,
   ragged, raggedy, ramshackle, ravaged, ravaging, resolvent, ruined,
   ruining, run-down, rusty, scraggly, seedy, self-destructive,
   separative, shabby, shattering, shoddy, slack, slatternly,
   slipshod, sloppy, slovenly, slummy, sluttish, solvent, sordid,
   spoiled, squalid, stale, subversionary, subversive, suicidal,
   tacky, tattered, time-scarred, timeworn, tottery, toxic, tragic,
   tumbledown, undone, unkempt, unneat, unsightly, untidy, vandalic,
   vandalish, vandalistic, wasted, wasteful, wasting, withering, worn,
   wrecked, wreckful