Where does the expression “to hold your horses” come from and What does it mean?

Originally, the expression “to hold one’s horses” had nothing but its literal meaning, to keep one’s team of horses from getting excited.

In the United States somewhat more than a hundred years ago, however, the expression began to be applied to people, always as a mild adjuration.

And because of an assumed rustic source, the suggestion that one control one’s temper or patience is generally phrased, placatingly, “Now, just hold your hosses.”