The Collaborative International Dictionary of English v.0.48:

Inalterable \In*al"ter*a*ble\, a. [Pref. in- not + alterable:
   cf. F. inalt['e]rable.]
   Not alterable; incapable of being altered or changed;
   unalterable. -- In*al"ter*a*ble*ness, n. --
   In*al"ter*a*bly, adv.
   [1913 Webster]