The Collaborative International Dictionary of English v.0.48:

Unpardonable \Unpardonable\
   See pardonable.

The Collaborative International Dictionary of English v.0.48:

Unpardonable \Un*par"don*a*ble\, a.
   Not admitting of pardon or forgiveness; inexcusable.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

unpardonable
    adj 1: not admitting of pardon; "unpardonable behavior" [ant:
           pardonable]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "unpardonable":
   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, unforgivable, unjustifiable,
   unreasonable, unspeakable, untenable, unwarrantable, unworthy,
   vicious, vile, villainous, wicked, wrong