The Collaborative International Dictionary of English v.0.48:

Dumbness \Dumb"ness\, n.
   The quality or state of being dumb; muteness; silence;
   inability to speak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dumbness
    n 1: the quality of being mentally slow and limited [syn:
         denseness, dumbness, slow-wittedness]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "dumbness":
   anaudia, aphasia, aphonia, asininity, blockishness, boobishness,
   boorishness, bovinity, brevity, briefness, brusqueness,
   cloddishness, conciseness, crassness, crudeness, curtness,
   deaf-muteness, denseness, density, dim-wittedness, dimness,
   doltishness, dull-wittedness, dullardism, dullness, duncery,
   economy of words, grossness, hebetude, inarticulateness,
   laconicalness, laconism, lethargy, loutishness, lumpishness,
   muteness, mutism, ninnyism, oafdom, oafishness, obmutescence,
   obtuseness, opacity, pauciloquy, quietness, quietude, shortness,
   silence, simpletonianism, slowness, sluggishness, sottishness,
   sparingness of words, speechlessness, stolidity, stupidity,
   taciturnity, terseness, thick-headedness, thick-wittedness,
   tonguelessness, unloquaciousness, untalkativeness, unteachability,
   voicelessness, wordlessness, wrongheadedness, yokelism