Modern set phrase popularized through 20th-century literature - appears in Zhang Yang's novel 第二次握手 ('The Second Handshake'). Built from classical components: 忠贞 'loyal and chaste' + 不渝 'does not change.'
usage
Used both for political/national loyalty and for romantic faithfulness. 渝 here means 'to change/break' - its only common modern use; outside this idiom you mostly see 渝 as the abbreviation for Chongqing 重庆.
