The Collaborative International Dictionary of English v.0.48:

Indeterminable \In`de*ter"mi*na*ble\, a. [L. indeterminabilis:
   cf. F. ind['e]terminable. See In- not, and Determine.]
   Not determinable; impossible to be determined; not to be
   definitely known, ascertained, defined, or limited. --
   In`de*ter"mi*na*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indeterminably
    adv 1: in an indeterminable manner; "their relationship was of
           an indeterminably vague nature"