The Collaborative International Dictionary of English v.0.48:

Inexpiable \In*ex"pi*a*ble\, a. [L. inexpiabilis: cf. F.
   inexpiable. See In- not, and Expiable.]
   [1913 Webster]
   1. Admitting of no expiation, atonement, or satisfaction; as,
      an inexpiable crime or offense. --Pomfret.
      [1913 Webster]

   2. Incapable of being mollified or appeased; relentless;
      implacable. [Archaic] "Inexpiable hate." --Milton.
      [1913 Webster]

            They are at inexpiable war with all establishments.
                                                  --Burke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inexpiable
    adj 1: incapable of being atoned for