knowyourphrase.com - Meanings and Origins of Phrases and Sayings!
Note: Tracking the origins of phrases is impossible most of the time! Mostly, what you'll find are the popular theories as to how an idiom originated, but it may or may not be necessarily so.
In addition, quotes that contain a particular phrase may be taken from old newspapers, poems, or books that were written centuries ago, but this by no means confirms that the phrase originates from said newspapers, poems, or books. In all likelihood, if an expression is being used in a newspaper, it's probably already a well known saying and is thus, from an older time. Still, old quotes give an indication on how old a saying is, which is pretty cool!
When you receive a gift from someone, do not be ungrateful.
Imagine someone receiving a gift, but instead of being thankful, they start analyzing it for flaws. The person who gave the gift may see this as rude behavior and take offense. Hence, this saying is typically directed towards a person who is guilty of this sort of behavior.
Never look a gift horse in the mouth! Wait, why would I not want to do that? Well, think of it this way: if someone was polite enough to give you your very own horse as a gift, would you then inspect the animal to see if it's of good quality, especially when the person who gave the horse to you is standing right there next to you? Probably not, because that might be seen as very rude behavior.
Apparently, though, there were people who would inspect the horse, particularly the mouth region, in order to determine the horses age. Yeah, you can do that. The length of a horses teeth can be used as an indicator for their age; the longer the teeth, the older the horse.
This phrase goes back to at least the 16th century. For example, John Heywood, believed to have lived from the years 1497 - 1580 C.E., wrote following in a book of his:
"Where gifts be given freely—east, west, north or south—
No man ought to look a given horse in the mouth."
According to Wikitionary, Eusebius Jerome, who lived around 346 - 420 C.E., may have also used the phrase. If so, this saying would be much older than the above example from John Heywood.
* I prefer to wear light colored shirts, but my mom gave me a bunch of dark colored ones; oh well, I shouldn't look a gift horse in the mouth and will try them on later.