(US, UK, idiomatic) imprisoned with little or no chance of escape; locked up.
If it was him we can have him under lock and key in 24 hours.
(US, UK, idiomatic) safely guarded; locked away.
Source: en.wiktionary.orgA man who is "of sound mind" is one who keeps the inner madman under lock and key. Paul Valéry
.. Here I am shut up for the livelong day under lock and key and with keepers in a cell.... I will not deny that I would rather have died than have caused and suffered such trouble. Vincent van Gogh
The Bible is most dangerous book ever written on earth, keep it under lock and key. George Bernard Shaw
However, as this would allow virtually anyone to go through the gate, a fail-secure design is used: In a power outage, the gate can only be opened by a hand crank that is usually kept in a safe area or under lock and key. Source: Internet
A scrapbook, which travelled with Munson when she was committed, still resides under lock and key at the asylum. Source: Internet
As a result, the government has taken all the children and is keeping them under lock and key, fearful of their potential. Source: Internet