noun
the state or quality of being unfaithful; disloyalty or lack of faithfulness
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The unfaithfulness of the employee was discovered through careful monitoring of their activities.
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She couldn't believe the unfaithfulness of her best friend, who had been lying to her for months.
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I can't believe the unfaithfulness of that guy, he's been two-timing his girlfriend for weeks.
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The unfaithfulness of the weather forecast left us unprepared for the sudden storm.