Why do we say "Don't look a gift horse in the mouth"?

Meaning:

If you are given something for free don't find fault with it or examine it too

Background:

The reference is to the habit of checking a horses age by looking at its teeth. The longer the teeth the older the horse.

Early Use:

The earliest recorded use is in a work by St. Jerome in A.D. 420 written in Latin.

'Equi donati dentes non inspiciuntur'.

It is also found in Russian and German texts.

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