The Collaborative International Dictionary
Perfidiously
Perfidiously \Per*fid"i*ous*ly\, adv. In a perfidious manner.
Wiktionary
perfidiously
adv. In a perfidious manner.
WordNet
perfidiously
adv. in a perfidious manner; "he was playing perfidiously one side against the other"
Usage examples of "perfidiously".
But the same could not be said of the English nation, which had in his view most foully apostatized from its pure creed, and most perfidiously betrayed the high commission it had received from Heaven.