The Collaborative International Dictionary of English v.0.48:

Unexpressive \Un`ex*press"ive\, a.
   1. Not expressive; not having the power of utterance;
      inexpressive.
      [1913 Webster]

   2. Incapable of being expressed; inexpressible; unutterable;
      ineffable. [Obs.]
      [1913 Webster]

            Run, run, Orlando; carve on every tree
            The fair, the chaste and unexpressive she. --Shak.
      [1913 Webster] -- Un`ex*press"ive*ly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unexpressive
    adj 1: deliberately impassive in manner; "deadpan humor"; "his
           face remained expressionless as the verdict was read"
           [syn: deadpan, expressionless, impassive, poker-
           faced, unexpressive]