Don't look a gift-horse in the mouth.
What is a gift-horse? Why shouldn't you look in its mouth?
What does this idiom actually mean and how is it used?
A horses teeth are regarded as a good guide to its age. When you buy a horse you check its teeth to see if they match the age of the horse according to the seller.
If someone gives you a horse as a gift, it is considered ungrateful to check its teeth. You are pointedly drawing attention to your doubts about the quality of the gift.
Don't be ungrateful when you receive a gift.
here's the origin:
As with most proverbs the origin is ancient and unknown. We have some clues with this one however. The phrase was originally "don't look a given horse in the mouth" and first appears in print in 1546 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."
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 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 descendents [sic].
here's an alternative explanation, from the question Does the phrase looking a gift horse in the mouth originate from the legend of the Trojan Horse?
No - you can estimate a horse's age by looking at its teeth. Looking a "gift horse in the mouth" would be like judging the gift's value or appearing ungrateful. The general idea is: it may not be ideal (like an old horse) but it was free & you can still make good use of it & be grateful for having one at all.
It is rude to be critical of a gift. Traditionally, one checks the health of a horse by examining its mouth. (Serial numbers are often tattooed on the inner lip of a horse, for tracking reasons, too.) Therefore, looking a gift horse in the mouth means you are critiquing the quality of the horse given to you.
A modern example: receiving an iPod as a gift, and then complaining that it doesn't have the memory capacity you wanted, or that you wanted a Zune, instead.