The Collaborative International Dictionary of English v.0.48:

Unforgivable \Unforgivable\
   See forgivable.

WordNet (r) 3.0 (2006):

unforgivable
    adj 1: not excusable [syn: inexcusable, unforgivable]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "unforgivable":
   abominable, arrant, atrocious, bad, base, black, blamable,
   blameworthy, criminal, damnable, dark, disgraceful, evil,
   execrable, flagitious, flagrant, foul, heinous, improper,
   indefensible, inexcusable, inexpiable, infamous, iniquitous,
   irremissible, knavish, low, monstrous, naughty, nefarious, peccant,
   rank, reprehensible, reprobate, scandalous, shameful, sinful,
   unallowable, unconscionable, unjustifiable, unpardonable,
   unreasonable, unspeakable, unwarrantable, unworthy, vicious, vile,
   villainous, wicked, wrong