The Collaborative International Dictionary of English v.0.48:

Damnable \Dam"na*ble\, a. [L. damnabilis, fr. damnare: cf. F.
   damnable. See Damn.]
   1. Liable to damnation; deserving, or for which one deserves,
      to be damned; of a damning nature.
      [1913 Webster]

            A creature unprepared unmeet for death,
            And to transport him in the mind he is,
            Were damnable.                        --Shak.
      [1913 Webster]

   2. Odious; pernicious; detestable.
      [1913 Webster]

            Begin, murderer; . . . leave thy damnable faces.
                                                  --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

damnable
    adj 1: deserving a curse; "her damnable pride" [syn: damnable,
           execrable]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "damnable":
   abhorrent, abominable, absolute, accursed, arrant, atrocious,
   awful, bad, base, black, blamable, blamed, blameworthy, blasted,
   blessed, complete, criminal, cursed, cussed, damn, damned, dark,
   darn, demoniac, demonic, despicable, detestable, devilish,
   diabolic, disgraceful, downright, dreadful, evil, execrable,
   fiendish, flagitious, flagrant, foul, ghoulish, goddamn, goddamned,
   gross, hateful, heinous, hellish, hideous, horrible, horrid,
   improper, infamous, infernal, iniquitous, knavish, loathsome, low,
   malevolent, malicious, monstrous, naughty, nefarious, odious,
   offensive, out-and-out, outrageous, outright, peccant, pernicious,
   rank, reprehensible, reprobate, rotten, satanic, scandalous,
   shameful, sinful, terrible, unforgivable, ungodly, unmitigated,
   unpardonable, unspeakable, unworthy, vicious, vile, villainous,
   wicked, wrong