The proverb refers to checking the age of horse by looking at the horse's teeth. The origin of the proverb is unknown. There are some clues to its origin though, as it appears in print in English in 1546, as "don't look a given horse in the mouth", in John Heywood's A Dialogue conteinyng the nomber in effect of all the Prouerbes in the Englishe tongue , where he gives it as:
"No man ought to looke a geuen hors in the mouth."
It is probable that 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.
So what does this have to do with the cache? I received a bunch of cache containers over the course of several recent events. This is one of them, and rather than taking up space in the garage, I actually put it out.
Good luck, and bring a writing utensil.