Source : Webster's Revised Unabridged Dictionary (1913)

Incommutable \In`com*mut"a*ble\, a. [L. incommutabilis: cf. F.
   incommutable. See {In-} not, and {Commutable}.]
   Not commutable; not capable of being exchanged with, or
   substituted for, another. --Cudworth. --
   {In`com*mut"a*ble*ness}, n. -- {In`com*mut"a*bly}, adv.

Source : WordNet®

incommutable
     adj 1: not subject to alteration or change [ant: {commutable}]
     2: not interchangeable or able to substitute one for another;
        "a rare incommutable skill"