He has always been extremely careful in everything he does.
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谨 is a formal/literary character. In everyday Chinese, you'll encounter it in compounds like 谨慎 (cautious) and 严谨 (rigorous). Standalone 谨 appears mainly in formal letters and dedications.
The speech radical on the left tells us 谨 is about words spoken with care - guarded, weighed, respectful. It indexes 谨 with the speech family: 说 say, 话 word, 谢 thank, 谅 forgive, 谋 plan - all verbs of measured communication.
堇 supplies the sound jǐn unchanged. It originally pictured a person scorched by fire over clay - a parched, scarce quality. That sense of scarcity gives a faint semantic flavour of restraint and frugality, matching 谨慎 cautious. Same phonetic in 勤 diligent, 仅 only, 谨 itself.
