The phrase "bad money drives out good" refers to the idea that when there is both good and bad currency circulating, people will hoard the good currency and spend the bad currency. This means that eventually, only the bad currency will be in circulation, driving out the good. There are several other terms that can express this concept, such as Gresham's law, which is a more formal version. Other equivalent phrases include "inferior money drives out superior," "debased money chases out pure," and "counterfeit currency diminishes the worth of genuine money." All of these terms convey the same message about the negative effects of having bad currency in circulation.