Phraseological units known among the people are sometimes very difficult to interpret, and the expression "lies like a gray gelding" is no exception. To an uninitiated person in the intricacies of the ancient Russian dialect, it is problematic to explain how a gelding (in a simple sense - a castrated stallion) can lie, why he is gray, and not gray or brown, and so on. Many experienced philologists argue that the occurrence of the phrase is associated with a gross mistake that distorted its meaning. But is it really so?
Dahl himself argued that the word "lying", included in the phraseological unit, is used incorrectly here. It came to replace the word "rushing", which is more optimal for the designation of any action performed by an animal. Initially, geldings were famous for their endurance and strength, which allowed them to be used in agriculture for cultivating private land. Young males did the job correctly and clearly followed the commands, but the older geldings, due to deteriorating health, made a crooked furrow. That is why it used to be said "rushing like a gray gelding", which meant making gross mistakes while performing work. Of course, this phrase is very difficult to attribute to its more modern counterpart, but it is precisely this mistake that the famous Russian writer claims.
The second interpretation of the origin of the phraseological unit is based on one person who lived several centuries ago. His name was Sivens-Mehring, and he became famous as a brazen and shameless liar. By old age, a man, apparently, completely ceased to distinguish between truth and lies, so for other people he became the standard of a liar, whose name label was applied to everyone who was caught in a lie. “He's lying like Sivens-Mehring” - you must admit that it sounds very similar. Perhaps, over time and as a result of some changes in pronunciation, this phrase acquired the form that we are seeing now.
And finally, one more version. An elderly person who boasted of his exploits on the love front or simply fell into insanity was previously jokingly nicknamed the gray gelding. After all, the word gray is a synonym for the word "gray". Another aphorism speaks in favor of this version - "bullshit", which has the same meaning.
An unambiguous answer about the reliable origin of the phrase "lies like a gray gelding" modern science of phraseological units cannot give us. It remains for us only, based on our own opinion, to interpret this expression, applying it in certain situations. Do gray geldings deserve such a rough comparison with those who constantly lie? Who knows.