Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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