This expression, generally used to caution people against being ungrateful and impolite when receiving a gift, has a long history.
It has been suggested to me that this expression has something to do with the Trojan horse, perhaps as a caution not to look in its mouth and see the soldiers hidden inside, who might then attempt to kill you before you could give warning. While I can see the appeal of this idea, unfortunately, it is incorrect. The expression associated with the Trojan War is “Beware of Greeks bearing gifts”.
Instead, the origin of this phrase is very simple. If you look at a horse’s teeth, (and you know what you are doing!) you can work out how old it is, and presumably get an idea of how much it is worth. Just as today you don’t immediately examine your present to see if there is a price tag on it, inspecting the teeth of a horse that’s been given to you would therefore be very rude and ungrateful.
So where does the expression come from? Well, we can trace it in English back to 1546, in “A dialogue conteinyng the nomber in effect of all the prouerbes in the Englishe tongue”, by John Heywood, where he lists it as “No man ought to looke a geuen hors in the mouth.”
Do we know where Heywood got it from? Well, most of his sayings were simply taken from the common tongue spoken around him, and there is no reason to suppose that this one wasn’t also in common use at the time. However, there is one further reference for the phrase, this time in Latin, in The Letter to the Ephesians, written by St. Jerome around 400 CE. It reads as follows: ‘Noli equi dentes inspicere donati‘, which translates as “never inspect the teeth of a gift horse”. What we don’t know is whether Jerome was just giving advice in a colourful way or whether it was an expression used around him at that time.
Whatever the origin beyond Jerome, it is a nice expression, adding colour to our language, and it seems a pity to talk about examining price tags or just being ungrateful!
This expression is by no means unique to English:
In Spanish it is the same: a caballo regalado no se le miran los dientes, while in German we find: Dem geschenkten Gaul schaut man nicht ins Maul.
In Welsh, the idiom goes with counting teeth: cyfrif dannedd ceffyl rhodd – “count a gift horse’s teeth”; an accurate description of what you might be doing if you look in its mouth!
Finally, it is interesting to note that there is a very similar expression, but with a different animal, in Greek: κάποιου χάριζαν γάιδαρο κι αυτός τον κοίταξε στα δόντια – “They gave someone a donkey, and he looked at its teeth”.