don't look a gift horse in the mouth
Since horses' gums recede as they age, which makes the teeth appear to grow long, checking the teeth of a horse given as a gift is a way of checking for old age.
- He ne’er consider'd it, as loth
- To look a Gift-horse in the mouth.
Although uncertain, the origin can be traced even further to St. Jerome's "Equi donati dentes non inspiciuntur.", The Letter to the Ephesians, circa AD 400.
- Gregory Y. Titelman, Random House Dictionary of Popular Proverbs and Sayings, 1996, ISBN 0-679-44554-4, p. 69.