The Collaborative International Dictionary of English v.0.48:

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.
   Incompact