As per Wikipedia: "Hold your horses", sometimes said as "Hold the horses", is a common idiom to mean "hold on" or wait, which is believed to have originated in the United States of America in the 19th century and is historically related to horse riding, or driving a horse-drawn vehicle.

I haven't the slightest idea..I do not own a horse and dang sure ain't trying to lift one...

Meaning - Hold on; be patient. Origin: US origin - 19th century. In keeping with its American origin, it originally was written 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)" http://www.phrases.org.uk/meanings/184400.html "HOLD YOUR HORSES - "Take it easy; keep calm; don't do anything rash. It is what one had to do with horses when they began to get nervous or excited; by 1844 it had been extended to people, as in the 'New Orleans Picayune': 'Oh, hold your horses, Squire, There's no use getting' riled, no how.'" From "The Dictionary of Cliches" by James Rogers (Ballantine Books, New York, 1985). Something else that might be said in the paragraph is that "Hold your horses" is among a surprising number of horsy phrases surviving long past the time when most people traveled by horse or horse-drawn vehicle. Another example: "stiff-necked," which somebody asked about today, a few lines up. http://www.phrases.org.uk/bulletin_board/17/messages/286.html Farm horses are not particularly excitable. You would hold them if you wished them NOT to begin plowing or whatever. Where I come from "Hold your horses" means "Wait just a damned minute!" The 1844 quotation sounds to me atypical. Perhaps the phrase was used differently in New Orleans, or perhaps it means "Wait, Squire, don't go off half-cocked." http://www.phrases.org.uk/bulletin_board/29/messages/211.html Idioms: hold one's horses Slow down, be patient, as in Dad told Kevin to hold his horses on Christmas shopping, since it was only July, or Hold your horses, I'm coming. This expression alludes to a driver making horses wait by holding the reins tightly. [Slang; c. 1840] "Hold your horses", sometimes said as "Hold the horses", is a common idiom to mean "hold on" or wait, which is believed to have originated in the United States of America in the 19th century and is historically related to horse riding, or driving a horse-drawn vehicle. Use 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." http://www.answers.com/hold%20your%20horses http://idioms.thefreedictionary.com/hold+water