Source : Webster's Revised Unabridged Dictionary (1913)

Dumbly \Dumb"ly\, adv.
   In silence; mutely.

Source : WordNet®

dumbly
     adv 1: in a stupid manner; "he had so rapaciously desired and so
            obtusely expected to find her alone" [syn: {densely},
            {obtusely}]
     2: in an inarticulate manner; "I nodded dumbly and he slit the
        envelope"