The Collaborative International Dictionary of English v.0.48:

Unprobably \Un*prob"a*bly\, adv. [Pref. un- not + probably.]
   Improbably.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unprobably \Un*prob"a*bly\, adv. [Un- + L. probabilis
   approvable, fr. probare to approve. Cf. Probable.]
   In a manner not to be approved of; improperly. [Obs. & R.]
   [1913 Webster]

         To diminish, by the authority of wise and knowing men,
         things unjustly and unprobably crept in. --Strype.
   [1913 Webster]