From Longman Dictionary of Contemporary Englishunder lock and keyunder lock and keya) SHUT/CLOSEkept safely in a box, cupboard etc that is locked Dad keeps all his liquor under lock and key. b) KEEP somebody IN A PLACEkept in a place such as a prison → lock
Examples from the Corpus
under lock and key• Wherever they are kept, they should be out of reach of children and, where appropriate, under lock and key.• Your master should really have kept the book under lock and key.• Smith's copies have spent the last six days under lock and key at its Dunstable depot.• Since then, that length of self-healing cable has been kept under lock and key at the railway inspectorate building at Reading.• Oswald's FBI file has been kept under lock and key.• The older children were no longer kept under lock and key.• If they had kept me under lock and key from my fifteenth birthday until my twentieth, I might have escaped.• With Petersen under lock and key, life for the gumshoes of the Office of Security returned to normal. 