The phrase a dime a dozen refers to something very plentiful, common, and therefore, inexpensive. ... A phrase that began as a way to tout good value for the money evolved into a phrase that means something nearly worthless by virtue of its commonness and easy availability.

What does dime in a dozen mean?

So plentiful as to be valueless. For example, Don't bother to buy one of these—they're a dime a dozen. The dime was declared the American ten-cent coin in 1786 by the Continental Congress. [

Is dime a dozen an insult?

“Dime a dozen” origins

Over time, the idiom evolved to mean the opposite. Instead of something being a good deal, it became a phrase to describe something that's not valuable and easily available.