"Guarding under seven keys" is a popular expression in the Portuguese language, used in the sense of "something that is very well protected" or “a very well kept secret”.
According to historical records, this expression would have originated from a very common habit among the royalty of Portugal, during the 13th century.
All jewelry, documents and other objects of importance to the Portuguese Crown were kept in a special chest that had four different locks.
The four keys, which opened the locks, were given to four high-responsible Kingdom officials, requiring the presence of all four together for the chest to be opened.
At that time, this was considered one of the safest ways to guard treasures and secret information.
Over time, the act of storing something with multiple keys has become synonymous with security.
Example: "I'm thinking of moving house, but keep this under lock and key!”
The number seven, however, was incorporated into the expression because it has a kabbalistic and mystical meaning for some ancient religions, mainly among the Babylonians and Egyptians.
In English, the expression “keep under lock and key” does not have a literal translation, but it can be replaced by the phrase under lock and key, which has a similar meaning to the popular expression in Portuguese.