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):

speechless
    adj 1: temporarily incapable of speaking; "struck dumb";
           "speechless with shock" [syn: speechless, dumb]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "speechless":
   anaudic, aphasic, aphonic, breathless, brief, brusque, close,
   close-tongued, closemouthed, concise, curt, dazed, dumb,
   dumbfounded, dumbstricken, dumbstruck, economical of words,
   inarticulate, indisposed to talk, laconic, mum, mute, nonplussed,
   quiet, reserved, shocked, short, silent, snug, sparing of words,
   stricken dumb, taciturn, terse, thunderstruck, tight-lipped,
   tongue-tied, tongueless, uncommunicative, unloquacious,
   untalkative, voiceless, word-bound, wordless