TREACHEROUSLY
faithlessly, traitorously, treacherously, treasonably, false
(adverb) in a disloyal and faithless manner; “he behaved treacherously”; “his wife played him false”
Source: WordNet® 3.1
Etymology
Adverb
treacherously (comparative more treacherously, superlative most treacherously)
In a treacherous manner.
Source: Wiktionary
TREACHEROUS
Treach"er*ous, a. Etym: [See Treacher.]
Definition: Like a traitor; involving treachery; violating allegiance or
faith pledged; traitorous to the state or sovereign; perfidious in
private life; betraying a trust; faithless.
Loyal father of a treacherous son. Shak.
The treacherous smile, a mask for secret hate. Cowper.
Syn.
– Faithless; perfidious; traitorous; false; insidious; plotting.
– Treach"er*ous*ly, adv.
– Treach"er*ous*ness, n.
Source: Webster’s Unabridged Dictionary 1913 Edition