The Collaborative International Dictionary of English v.0.48:

Illimitable \Il*lim"it*a*ble\, a. [Pref. il- not + limitable:
   cf. F. illimitable.]
   Incapable of being limited or bounded; immeasurable;
   limitless; boundless; as, illimitable space.
   [1913 Webster]

         The wild, the irregular, the illimitable, and the
         luxuriant, have their appropriate force of beauty. --De
                                                  Quincey.

   Syn: Boundless; limitless; unlimited; unbounded;
        immeasurable; infinite; immense; vast. --
        Il*lim"it*a*ble*ness, n. -- Il*lim"it*a*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

illimitable
    adj 1: without limits in extent or size or quantity; "limitless
           vastness of our solar system" [syn: illimitable,
           limitless, measureless]