In a deceitful, disloyal, or insincere manner; with false or treacherous intentions.
From false-hearted + -ly (adverb suffix). The adverbial form describing the manner of acting with a false heart or dishonest intentions.
This adverb is deliciously specific—it doesn't just mean doing something dishonest, but doing it while pretending to be genuine. It captures the performance aspect of betrayal.
Complete word intelligence in one call. Free tier — 50 lookups/day.