FAITHLESSLY
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
faithlessly (comparative more faithlessly, superlative most faithlessly)
In a faithless manner
Source: Wiktionary
FAITHLESS
Faith"less, a.
1. Not believing; not giving credit.
Be not faithless, but believing. John xx. 27.
2. Not believing on God or religion; specifically, not believing in
the Christian religion. Shak.
3. Not observant of promises or covenants.
4. Not true to allegiance, duty, or vows; perfidious; trecherous;
disloyal; not of true fidelity; inconstant, as a husband or a wife.
A most unnatural and faithless service. Shak.
5. Serving to disappoint or deceive; delusive; unsatisfying. "Yonder
faithless phantom." Goldsmith.
– Faith"less*ly, adv.Faith"less*ness, n.
Source: Webster’s Unabridged Dictionary 1913 Edition