Look a gift horse in the mouth

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.

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