“Hold your horses” is a very popular English idiomatic expression. It has nothing to do with horses, even though it’s related to its origin. It means “hold on” or wait, which is believed to have originated in the United States in the XIX century and is historically related to horse riding, or driving a horse-drawn vehicle.
It was written originally as “hold your hosses” and it appears in print that way many times from 1844 onwards. In Picayune (New Orleans) September 1844, we have: “Oh, hold your hosses, Squire. There’s no use gettin’ riled, no how.”
It’s clear that “hoss” is the US slang term for horse, which was certainly known by 1844, as in David Humphreys’ The Yankey in England, 1815: “The boys..see a ghost in the form of a white hoss; and an Indian in every black stump.”
It isn’t until much later, in Chatelaine, 1939, we get the more familiar phrase: “Hold your horses, dear.” In 1943 there’s a more descriptive use, in Hunt and Pringle’s Service Slang: “Hold your horses, hold the job until further orders. (comes from the Artillery)”
The phrase is typically used when someone is rushing in to something. It is often combined with linked idioms such as , “cool your jets,” or “look before you leap.” However it also has a more literal meaning and in certain circumstances is the preferred idiom to use.
“Hold your horses” literally means to keep your horse (or horses) still, which would be used when horse riding, or driving a horse-drawn vehicle. Thus it is very easy for someone without previously hearing the expression to understand its meaning. Someone is to wait for a moment. It is usually followed up with an explanation to demonstrate why they should wait. For example, “Hold your horses, you haven’t thought about this yet.”
When your boss is bugging you to turn in a report, you can calmly say, “Hold your horses boss, the report has been on your desk since 8:00 a.m.” He’ll understand and leave you alone.