(keep something/put something/be) under lock and key phrase
To keep something in a safe place such as a box, room, or bank
My jewelry is securely under lock and key.
My grandfather kept his testament under lock and key.
I put all of my money under lock and key at the bank.
To keep someone in prison
Finally the criminal was kept under lock and key.
The police have catched the escaped prisoner and put him under lock and key.
1. The act of keeping current customers
2. Used to indicate things you want to keep
To not react or deal with something that is wrong or unpleasant
Keep someone or something in a small space
Hold the position for someone or take their responsibility until they are back
To keep memory in a particular place in one's heart or mind.
The verb "keep/put" should be conjugated according to its tense.