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From the Book of Changes (《周易·乾》): 君子终日乾乾，夕惕若厉，无咎 - 'the gentleman strives all day long, vigilant by evening as if facing danger; no blame.' Often used in imperial edicts and self-evaluations as a stock phrase of dutiful officials.
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Highly literary. Reserve for written or formal speech, especially praise of officials, leaders, or ancestors. The Yongzheng Emperor famously misquoted this phrase as 夕惕朝乾 in a memorial response, generating decades of historical gossip about whether it was a slip or a snub.
