Source : Webster's Revised Unabridged Dictionary (1913)

Unexpressive \Un`ex*press"ive\, a.
   1. Not expressive; not having the power of utterance;
      inexpressive.

   2. Incapable of being expressed; inexpressible; unutterable;
      ineffable. [Obs.]

            Run, run, Orlando; carve on every tree The fair, the
            chaste and unexpressive she.          --Shak.
      -- {Un`ex*press"ive*ly}, adv.

Source : WordNet®

unexpressive
     adj : deliberately impassive in manner; "deadpan humor"; "his face
           remained expressionless as the verdict was read" [syn:
           {deadpan}, {expressionless}, {impassive}, {poker-faced}]