PERFIDIOUSLY
perfidiously
(adverb) in a perfidious manner; āhe was playing perfidiously one side against the otherā
Source: WordNet® 3.1
Etymology
Adverb
perfidiously (comparative more perfidiously, superlative most perfidiously)
In a perfidious manner.
Source: Wiktionary
Per*fid"i*ous*ly, adv.
Definition: In a perfidious manner.
PERFIDIOUS
Per*fid"i*ous, a. Etym: [L. perfidious.]
1. Guilty of perfidy; violating good faith or vows; false to trust or
confidence reposed; teacherous; faithless; as, a perfidious friend.
Shak.
2. Involving, or characterized by, perfidy. "Involved in this
perfidious fraud." Milton.
Source: Webster’s Unabridged Dictionary 1913 Edition