Source : Webster's Revised Unabridged Dictionary (1913)

Inexcusably \In`ex*cus"a*bly\, adv.
   With a degree of guilt or folly beyond excuse or
   justification.

         Inexcusably obstinate and perverse.      --Jortin.

Source : WordNet®

inexcusably
     adv 1: in an unpardonable manner or to an unpardonable degree; "he
            was inexcusably cruel to his wife" [syn: {unpardonably},
             {unforgivably}] [ant: {excusably}, {excusably}, {excusably}]
     2: without any excuse; "he is unjustifiably harsh on her" [syn:
         {unjustifiably}] [ant: {justifiably}]