What is the origin of the idiom "a dime a dozen" and when to use it?

The idiom "a dime a dozen" has its origins in the United States and is believed to have emerged in the early 20th century. It reflects the idea that something is so common and abundant that its value is minimal, much like items that can be bought for a dime (10 cents) and are readily available in large quantities. This idiom is used to describe things that are easy to find, abundant, or unremarkable due to their ubiquity.