With the holidays upon us, suspicious gifts may arrive at your door. You might inspect a horse’s teeth to estimate its age and value. To do this to a horse that is a gift is ungrateful – although, I know something about what it costs to keep a horse, so a sad old plug is a gift you might refuse!

Knowyourphrase says:

John Heywood, who is thought to have lived from the years 1497-1580 C.E., is said to have wrote the saying in a book of his called A dialogue conteinyng the nomber in effect of all the prouerbes in the englishe tongue, 1546:
“Where gifts be given freely—east, west, north or south— No man ought to look a given horse in the mouth.“

Phrase Finder says “As with most proverbs the origin is ancient and unknown,” but also notes that Heywood collected exprssions in common use at the time, and didn’t claim to have coined them.

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.