What is the origin of the idiom "at the back of one's mind" and when to use it?

The idiom "at the back of one's mind" is derived from the idea that our thoughts and concerns occupy different positions in our mental space. The "back of one's mind" symbolizes a less prominent or less actively considered area of thought compared to the forefront of one's mind, where immediate or pressing matters reside. This expression is used to describe thoughts, ideas, or concerns that are not at the forefront of someone's thinking but are still present and lingering in their consciousness. It often implies that these thoughts are not immediately obvious or the primary focus of one's attention.