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, and instead of being thankful, they start
analyzing it for flaws to discern if it meets their standards. Wouldn't that show
a lack of respect towards the generous giver? Sure, thus, people might direct
this saying towards a person who is guilty of such 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.
According to Wikitionary, this phrase can be traced back to Eusebius Jerome, who lived in the 4th century CE. Apparently, Jerome had used this phrase in one of his writings, as he penned the words:
"Never inspect the teeth of a gift horse."
* 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.