What is the origin of the idiom put your money where your mouth is?

This idiom originates from the late 1700s. People would raise their money to their mouth to demonstrate that they were unsure if something they said or are about to say was/or will be true.

By raising their money to their mouth, people felt it would obscure their voice meaning they could backtrack on what they said and claim they said something else if their prediction didn't come to fruition.

The idiom is commonly used by fans of sport to demonstrate how unsure they are about the outcome of an game.