The Collaborative International Dictionary of English v.0.48:

Underhandedly \Un"der*hand`ed*ly\
   ([u^]n"d[~e]r*h[a^]nd`[e^]d*l[y^]), adv.
   In an underhand manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underhandedly
    adv 1: 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: underhandedly,
           underhand]