TREACHEROUSLY, adverb trech'erously. By violating allegiance or faith pledged; by betraying a trust; faithlessly; perfidiously; as, to surrender a fort to an enemy treacherously; to disclose a secret treacherously You treacherously practic'd to undo me.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI