Don't be ungrateful when you receive a gift.

Don't look a gift horse in the mouth

Meaning

Don't be ungrateful when you receive a gift.
Origin

Proverbs are 'short and expressive sayings, in common use, which are recognized as conveying some accepted truth or useful advice'. This example, also often expressed as 'never look a gift horse in the mouth', is as pertinent today as it ever was.

don't look a gift horse in the mouth As horses develop they grow more teeth and their existing teeth begin to change shape and project further forward. Determining a horse's age from its teeth is a specialist task, but it can be done. This incidentally is also the source of another teeth/age related phrase - long in the tooth.

The advice given in the 'don't look...' proverb is: when receiving a gift be grateful for what it is; don't imply you wished for more by assessing its value.

As with most proverbs the origin is ancient and unknown. We have some clues with this one however. The phrase appears in print in English in 1546, as "don't look a given horse in the mouth", in John Heywood's A dialogue conteinyng the nomber in effect of all the prouerbes in the Englishe tongue, where he gives it as:

"No man ought to looke a geuen hors in the mouth."

It is probable that Heywood obtained the phrase from a Latin text of St. Jerome, The Letter to the Ephesians, circa AD 400, which contains the text 'Noli equi dentes inspicere donati' (Never inspect the teeth of a given horse). Where St Jerome got it from we aren't ever likely to know.

Heywood is an interesting character in the development of English. He was employed at the courts of Henry VIII and Mary I as a singer, musician, and playwright. His Proverbs is a comprehensive collection of those sayings known at the time and includes many that are still with us:

- Many hands make light work
- Rome wasn't built in a day
- A good beginning makes a good ending

and so on. These were expressed in the literary language of the day, as in "would yee both eat your cake, and have your cake?", but the modern versions are their obvious descendants.

We can't attribute these to Heywood himself; he collected them from the literary works of the day and from common parlance. He can certainly be given the credit for introducing many proverbs to a wide and continuing audience, including one that Shakespeare later borrowed - All's well that ends well.
http://www.phrases.org.uk/meanings/dont-look-a-gift-horse-in-the-mouth.html