Refers to a lack of loyalty or faithfulness, often in personal relationships or duties. Somewhat formal or literary.
His unfaithfulness caused them to break up.
裏切り is a more common and direct word for betrayal or treachery, often used in everyday speech.
Compound of 不 (fu-, 'not') + 忠実 (chūjitsu, 'loyal, faithful'). The reading ふちゅうじつ follows standard on'yomi patterns.