Source : Webster's Revised Unabridged Dictionary (1913)

Expressionless \Ex*pres"sion*less\, a.
   Destitute of expression.

Source : WordNet®

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