The Collaborative International Dictionary of English v.0.48:

Speechless \Speech"less\, a.
   1. Destitute or deprived of the faculty of speech.
      [1913 Webster]

   2. Not speaking for a time; dumb; mute; silent.
      [1913 Webster]

            Speechless with wonder, and half dead with fear.
                                                  --Addison.
      [1913 Webster] -- Speech"less*ly, adv. --
      Speech"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

speechlessness
    n 1: the property of being speechless