The Collaborative International Dictionary of English v.0.48:

Inexcusable \In`ex*cus"a*ble\, a. [L. inexcusabilis: cf. F.
   inexcusable. See Excuse.]
   Not excusable; not admitting excuse or justification; as,
   inexcusable folly.
   [1913 Webster]

         Therefore thou art inexcusable, O man, whosoever thou
         art that judgest; for wherein thou judgest another,
         thou condemnest thyself; for thou that judgest doest
         the same things.                         --Rom. ii. 1.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inexcusable
    adj 1: without excuse or justification [ant: excusable]
    2: not excusable [syn: inexcusable, unforgivable]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "inexcusable":
   blamable, blameworthy, censurable, impermissible, indefensible,
   inexpiable, intolerable, irremissible, reprehensible, unallowable,
   unconscionable, unforgivable, unjustifiable, unjustified,
   unpardonable, unpermissible, unreasonable, untenable,
   unwarrantable