Meaning of “under lock and key” in the English Dictionary

british dictionary

"under lock and key" in British English

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under lock and key

locked away safely:

Her jewellery is securely under lock and key at the bank.

If a person, especially a criminal, is under lock and key, they are being kept in a place from which they cannot escape, usually a prison.

(Definition of “under lock and key” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)