What is the origin of the idiom "on the shelf" and when to use it?

The origin of the idiom "on the shelf" can be traced back to the practice of placing items, particularly books, on a shelf for storage or display. Over time, the phrase took on a figurative meaning when applied to people, particularly unmarried women, who were considered past the typical age for marriage. It is used to describe someone, especially a woman, who is not married and is seen as being too old to get married. the phrase "on the shelf" can be considered somewhat old-fashioned. It reflects traditional societal expectations and judgments related to age and marriage.