Slow down or stop, be patient, keep your shirt on, cool your jets.
The phrase “hold your horses” literally means to keep your horse (or horses) still. Therefore, it’s easy for anyone who has never heard the expression before to understand its meaning. It is usually followed up with an explanation of why you should wait. For example, “Hold your horses. We’re almost there.”
And those who aren’t in control of the horses often say, “Are we there yet?”
The phrase is historically related to horse riding, or driving a horse-drawn vehicle. A number of explanations, all unverified, have been offered for the origin of the phrase, dating back to usage in Ancient Greece.
In Book 23 of the Iliad, Homer says, “Hold your horses!” when referring to Antilochus driving like a maniac in a chariot race that Achilles initiates in the funeral games for Patroclus.
After the invention of gunpowder, the Chinese would have to hold their horses because of the noise. On the farm where I grew up, where we used both horses and tractors, I would sometimes have to hold the horses by their bridles and talk to them, so that they wouldn’t panic when the noisy tractor rolled by.
In 19th-century USA, where it was written as ‘hold your hosses’ (“hoss” being a US slang term for horse), it appears in print that way many times from 1843 onwards. Example: from Picayune (New Orleans) in September 1844, “Oh, hold your hosses, Squire. There’s no use gettin’ riled, no how.”
The term was also used in army artillery units. Example: Hunt and Pringle’s Service Slang (1943) quotes, “Hold your horses, hold the job until further orders.”
In the Arctic, perhaps they say, “Hold your dogs!”