Don't Look a Gift Horse in the Mouth
Definition: The unappreciative act of questioning or examining a gift too closely.
Origin
This expression is old — so old that its exact origin cannot be exhumed. St. Jerome of the 4th century wrote about this expression and he even considered it old at that time! The current thought is that the phrase originated because of the poor dental hygiene of horses. As horses age their teeth become yellow and their gums recede. Hence, like the rings of a tree, the age of a horse can be deduced by the condition of its teeth. So if you’re ever sold a “young” horse, but have suspicions, looking into its mouth may be impolite, but smart.
