Deceitful, disloyal, or treacherous in one's true feelings and intentions; not sincere or genuine.
From false + hearted (having a certain kind of heart/character). A compound adjective describing internal dishonesty, using 'heart' as the seat of emotions and truthfulness.
Shakespeare loved this word—it captures a specific kind of wickedness, someone who smiles while plotting against you. The compound 'false-hearted' is more cutting than just calling someone a liar.
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