What is the origin of the idiom "off one's rocker" and when to use it?

The origin of the idiom "off one's rocker" is not entirely clear, but it likely comes from the idea of a rocking chair. When someone is "off their rocker," it means they have lost their balance or stability, much like a rocking chair that is no longer balanced and wobbles unpredictably. It is used to describe someone who is behaving in a crazy, irrational, or mentally unstable manner.