insoluble

from WordNet (r) 3.0 (2006)

insoluble
    adj 1: (of a substance) incapable of being dissolved [syn:
           {insoluble}, {indissoluble}] [ant: {soluble}]
    2: admitting of no solution or explanation; "an insoluble doubt"
       [ant: {soluble}]
    3: without hope of solution; "an insoluble problem"

from The Collaborative International Dictionary of English v.0.48

Insoluble \In*sol"u*ble\, a. [L. insolubilis indissoluble, that
   can not be loosed: cf. F. insoluble. See {In-} not, and
   {Soluble}, and cf. {Insolvable}.]
   [1913 Webster]
   1. Not soluble; in capable or difficult of being dissolved,
      as by a liquid; as, chalk is insoluble in water.
      [1913 Webster]

   2. Not to be solved or explained; insolvable; as, an
      insoluble doubt, question, or difficulty.
      [1913 Webster]

   3. Strong. "An insoluble wall." [Obs.] --Holland
      [1913 Webster]

from Moby Thesaurus II by Grady Ward, 1.0

24 Moby Thesaurus words for "insoluble":
      coherent, cohesive, enwrapped in mystery, impartible, indefinable,
      indiscerptible, indissoluble, indissolvable, indivisible,
      inexplicable, inextricable, infrangible, infusible, inseparable,
      insolvable, mysterious, mystic, unaccountable, undefinable,
      undividable, unexplainable, unified, uninterpretable, unsolvable