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perfidious (adj) treacherous.
A person who is perfidious is treacherous. They cannot be trusted. They are disloyal. They cause harm to the people that trust them.
- History is full of examples of rulers brought to ruin by perfidious followers. The very followers whom the rulers trust betray them in the hour of need, and cause their downfall.
- John was the victim of his perfidious friend. Pretending to be friendly, he ditched John in the hour of need. He joined with others to ruin the career of John.