It's Roman slang: "Noli equi dentes inspicere donati". For example, you can find it in St. Jerome's Commentary on the Epistle to the Ephesians, written in the 5th century.

You look in a horse's mouth to determine its age and/or health. So if someone gives you a horse, and you look in it's mouth, it's like looking for a price tag. It's rude!
Lucius: I bought you this tunic in Thebes for your birthday.
Quintus: Thanks dude. Hey, look, the seam has really cheap thread.
Lucius: Noli equi dentes inspicere donati.
Quintus: Wha?
Lucius: It's Latin. Don't look a gift horse in the mouth.
Quintus. Oh yah, right. Mea culpa.
by Ed the Horse July 18, 2009
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to be critical of a present or gift
Despite thought put into the gift, he decided to look a gift horse in the mouth and commented on the cheapness of the gift
by stanzz October 26, 2006
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A gift horse "lays" gifts, much like the golden goose.
You don't want to know what it eats.
The modern interpretation is corrupted from the original 'given,' not 'gifted.'
To not examine a gift is to ignore subtle compliments or insults, and can be dangerous. Such was the case in the Trojan Horse.
"This roast is delicious. Do you want to go to the slaughterhouse after?" mused Betty.
"Don't look a gift horse in the mouth." quipped Martha.

"My girlfriend's taking me out, but I think she got the money from whoring at the truck stop," said Jose.
"Don't look a gift horse in the mouth," cautioned Miguel.

(On a chairlift) "Isn't it sad there are no tee pees or buffalo here?" wondered Jane.
"Don't look a gift horse in the mouth," snapped Stacie.
by Assy Cheeks November 15, 2007
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meaning if something is free dont find the flaws in it.

one way to tell a horses age is by looking at the wear on his teeth , if someone gives you a horse theres a good chance it is old and you wont get much use out of it.
sam got a bike for christmas , but it wasnt the one he asked for .... "dont look a gift horse in the mouth"
by the_colt23 April 22, 2011
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"Never look a gift horse in the mouth" or "long in the tooth" are both expressions we often hear but not quite comprehend what it really means. Many may interpret is as "don't question an intelligent person" or "don't question the value of a gift". The following are two types of meaning of the phrase.

"Don't check his age." Traditionally, horsemen use teeth as a guide to determine the horse's age. This practice is not foolproof but many factors contribute when evaluating a horse's age. Deciduous teeth (milk teeth) are grown shortly after birth. They're paler and shorter than adult teeth. They gradually shed by the age of five and are replaced by permanent teeth with a concave surface which are larger and darker in colour. At the age of ten , the Galvayne's groove begins to appear and by the age of fifteen the groove would have reached half way down the tooth. When the horse reaches its mid twenties, the Galvayne's groove begins to disappear and entirely vanish, when they are old.

a) Don't try to evaluate a gift
b) be appreciative of a gift regardless whether it's big or small, instead of wanting something big and better.
c) be grateful and accept what is given.
d) don't question what is given to you, accept it as it is.
Harry Styles: Don't evaluate a girl by the size of her diamond ring. The grading or ranking scale that its is measured against may not be of importance to her. She may have deliberately skipped Chem class knowing that she will fail the unit on the 4C's.

Pinto's father taught her a valuable lesson in life which is "Never look a gifted horse in the mouth."

Expecting a horse for her 12th birthday, Pinto was disappointed when her father Miguel gifted her a cow in a ribbon.
by Toilet paper pessimist February 4, 2018
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A saying when someone gives you something useful but you don't like it at all and you would like to have something better for you .

In the past, when buying a horse, people looked to see if it had healthy teeth.
Dad: „I will give you my little car for your birthday.“
Daughter: „But I don’t like it! I’d rather like a big car!“
Dad: „OH, c’mon! Don’t look a gift horse in the mouth!“
by Jay Gee Pee May 28, 2022
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When you buy a new style Dr. Pepper and she accuses you of cheating with her mate
I was walking your friend home and this is how you treat me so Don't look a gift horse in the mouth
by Jermaine Burrell August 30, 2023
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