fidelity

From Late Middle English fidelite (“allegiance, fealty; faithfulness, trust; agreement made in good faith”), from Old French fidélité (“faithfulness, fidelity”) (modern French fidélité), from Latin fidēlitās (“faithfulness, fidelity”), from fidēlis (“dependable, trustworthy; faithful, loyal”) + -tās (suffix forming feminine abstract nouns denoting a state of being). Fidēlis is derived from fidēs (“belief, faith; confidence, trust; loyalty, fidelity; etc.”) (from Proto-Indo-European *bʰidʰ-, the zero-grade of *bʰeydʰ- (“to convince; to trust”) (whence English bide)) + -ēlis (a variant of -ālis (suffix forming adjectives of relationship)). Doublet of fealty.