/ ˌɪnfɪˈdeləti; NAmE / noun
[ U , C ] ( pl. -ies ) the act of not being faithful to your wife, husband or partner, by having sex with sb else
SYN unfaithfulness :
marital infidelity
She could not forgive his infidelities.
OPP fidelity
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WORD ORIGIN
late Middle English (in the senses lack of faith and disloyalty ): from Old French infidelite or Latin infidelitas , from infidelis not faithful, from in- not + fidelis faithful (from fides faith, related to fidere to trust).