'infidelity' originates from Latin, specifically the word 'infidelitas,' where 'in-' meant 'not' and 'fidelitas' meant 'faithfulness.'

'infidelitas' transformed into the Old French word 'infidelité,' and eventually became the modern English word 'infidelity' through Middle English.

Initially, it meant 'lack of faith or belief,' but over time it evolved into its current meaning of 'unfaithfulness in a relationship.'