I think I would like to read a study of ancient idioms, or at the very least have a catalog of such idioms. I think a reference material of that sort would have real (though limited) applications for scholarship. It may be that one exists, and if it doesn't I certainly wish it did. (If nothing else, it would make a very nice coffee table book for intellectual elitists and passive-aggressive snobs.)
I am reading through the letters of Gregory Akindynos in conjunction with my work on fourteenth century hesychasm, and, in the course of one of his letters to an unknown recipient, he writes, "In fact, I beleive that a man would have to be made of stone, if after associating with you even briefly, he does not become your friend..." I was immediately struck by the fact that the phrase "would have to be made of stone" had translated so well across chronological and cultural boundaries such that it should seem right at home in a modern text. My immediate suspicion was, I admit, that the translator had taken liberties with the text and translated an ancient idiom into a modern one. To my shock and delight, when I checked the Greek "made of stone" was right where it ought to be.
At the same time, in the next letter in the collection, addressed to the hieromonk Gregory, Akindynos writes, "You seem to be Egypt to me, as the saying goes." The two idioms placed in such close proximity (in this collection) was striking to me. If I think about it, I realize that the first is based on analogies to permanent features of reality (viz. the coldness and hardness of stone) while the latter is attached to transient cultural attitudes (viz. contemporary perceptions about Egypt). Still, that some cultural idioms should remain fresh and relevant through time while others become totally incomprehensible intrigues me.
(In case you were wondering, Akindynos thought the phrase "You seem to be Egypt to me" sufficiently ambiguous that he needed to explain it: "for you rarely produce something, but what you do produce is always something noble." I have my fingers crossed that "Egypt to me" makes its way back into the repertoire of modern idiom.)