Source : Webster's Revised Unabridged Dictionary (1913)

Inexpressive \In`ex*press"ive\, a.
   1. Inexpressible. [R.]

   2. Without expression or meaning; not expressive; dull;
      unintelligent; as, an inexpressive countenance.

Source : WordNet®

inexpressive
     adj : not expressive; "an inexpressive face"