The Collaborative International Dictionary of English v.0.48:

Calamitous \Ca*lam"i*tous\, a. [L. Calamitosus; cf. F.
   calamiteux.]
   [1913 Webster]
   1. Suffering calamity; wretched; miserable. [Obs.]
      [1913 Webster]

            Ten thousands of calamitous persons.  --South.
      [1913 Webster]

   2. Producing, or attended with distress and misery; making
      wretched; wretched; unhappy. "This sad and calamitous
      condition." --South. "A calamitous prison" --Milton.

   Syn: Miserable; deplorable; distressful; afflictive;
        wretched; grievous; baleful; disastrous; adverse;
        unhappy; severe; sad; unfortunate. --
        Ca*lam"i*tous*ly, adv. -- Ca*lam"i*tous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

calamitous
    adj 1: (of events) having extremely unfortunate or dire
           consequences; bringing ruin; "the stock market crashed on
           Black Friday"; "a calamitous defeat"; "the battle was a
           disastrous end to a disastrous campaign"; "such
           doctrines, if true, would be absolutely fatal to my
           theory"- Charles Darwin; "it is fatal to enter any war
           without the will to win it"- Douglas MacArthur; "a
           fateful error" [syn: black, calamitous, disastrous,
           fatal, fateful]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "calamitous":
   afflictive, awful, baneful, black, cataclysmal, cataclysmic,
   catastrophic, consuming, consumptive, deadly, demolishing,
   demolitionary, depredatory, desolating, destroying, destructive,
   devastating, dire, disastrous, distressful, distressing, doomful,
   dreadful, fatal, fateful, fratricidal, grievous, heartbreaking,
   internecine, lamentable, nihilist, nihilistic, pernicious,
   ravaging, regrettable, ruining, ruinous, self-destructive,
   subversionary, subversive, suicidal, terrible, tragic, unfortunate,
   vandalic, vandalish, vandalistic, wasteful, wasting, withering,
   woeful, wreckful