What does "on the shelf" mean?

Inactive, not employed, as in With mandatory retirement at 65, many useful employees are put on the shelf. [Second half of 1500s]

In a state of disuse, as in We'll have to put her proposal on the shelf until we have more funds. [Late 1800s]

Without prospects of marriage. For example, After she broke her third engagement, her parents were sure she'd be on the shelf. This usage is always said of a woman and today considered offensive. It is probably obsolescent. [Early 1800s] All these usages allude to an article left on the shelf of a store, bookcase, or the like.