From The Collaborative International Dictionary of English v.0.48 :
Dumbly \Dumb"ly\, adv. In silence; mutely. [1913 Webster]From WordNet (r) 3.0 (2006) :
dumbly adv 1: in a stupid manner; "he had so rapaciously desired and so obtusely expected to find her alone" [syn: dumbly, densely, obtusely] 2: in an inarticulate manner; "I nodded dumbly and he slit the envelope"