I found it for you: $ TERM master key is also often used to describe lever lock key. This usage may come from the fact that the key is reassembled into a skeleton figure, and the top circle is circumscribed by a triangle at one third of the vertical length, similar to a stylized skull.

Because the original master key is not a single one, but as many accessories as possible are hung on the key, so that when it is not easy to use, it can be used to unlock the lock, and then it looks very like the shape of a "skeleton", so it is named.

I hope I can help you ~