Result from Foreign Dictionaries (2 entries found)
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Perfidiously \Per*fid"i*ous*ly\, adv.
In a perfidious manner.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
perfidiously
adv 1: in a perfidious manner; "he was playing perfidiously one
side against the other"