Wiktionary
a. 1 (context US UK idiomatic English) imprisoned with little or no chance of escape; locked up. 2 (context US UK idiomatic English) safely guarded; locked away.
Wikipedia
Under Lock and Key is the third studio album by heavy metal band Dokken, released on November 22, 1985 through Elektra Records. The album reached No. 32 on the U.S. Billboard 200 and remained on that chart for 67 weeks. Two singles also charted: "The Hunter" and "In My Dreams", both reaching No. 25 and 24 on Billboard's Mainstream Rock respectively, with "In My Dreams" at No. 77 on the Billboard Hot 100. Under Lock and Key was certified Gold on March 4, 1986 and Platinum on April 14, 1987.
Usage examples of "under lock and key".
Ruthven and Jean are safe and well -- your friends, Vyland, are under lock and key and talking their heads off to beat the rap.
Occasionally reports would come in from palace servants, who told of a mysterious woman being held in the east wing under lock and key.
Let's tell them we know about Stinnes, that we have him under lock and key and are giving him a bad time.
I had access to a copy of that once, and hear that you have one in your college library under lock and key.
Nobody could have guessed that these courtly elderly gentlemen had come to decide whether one of their fellow creatures was to be left to enjoy his freedom or put under lock and key.
His wives were all either closely guarded or kept under lock and key.
I've got the copies of his reports in my desk, under lock and key.
Moreover, they had stopped him from consulting the dubious old books on forbidden secrets that were kept under lock and key in a vault at the university library.
It was strongly built of rough-hewn logs, the proprietor rightly judging, in the unregenerate days of Jackman's Gulch, that hogsheads of brandy and rum were commodities which had best be secured under lock and key.
Nothing whatever is under lock and key and the whole town runs in and out of the place all the time.