It's when paper money becomes trash.

Maybe you've seen images depicting hyperinflation in Germany after World War I. The German government had printed so much money that it became worthless confetti. Technically, German merchants still accepted the currency, but it was impractical. For example, it would have required wheelbarrows full of paper money to buy a loaf of bread.

At the time, no one would bother to pick up money off the ground. It wasn't worth any more than the other crumpled pieces of paper on the street.

Today, there's a similar situation in the US. When was the last time you saw someone pick up a penny off the street? A nickel? A dime?

Nowadays, even bums often can't be bothered to pick up anything less than a quarter.

The US dollar has become so debased that these coins are essentially pieces of rubbish. They have little to no practical value.