Source : Webster's Revised Unabridged Dictionary (1913)

Underhandedly \Un"der*hand`ed*ly\, adv.
   In an underhand manner.

Source : WordNet®

underhandedly
     adv : slyly and secretly; "Mean revenge, committed underhand"-
           John Donne; "oldline aristocratic diplomats
           underhandedly undermined the attempt...to align Germany
           with the Western democracies"- C.G.Bowers [syn: {underhand}]