Which source? Again, I don’t know. Maybe Ta’amei Minhagim. If it’s a chassidishe minhag maybe chassidishe seform. Just (again, not you ?”?) don’t automatically run to the “seforim” of our neighbors’ local “beis medrash”.
How old do I think it is? Again, don’t know. Maybe at the same era the women refrained from sewing something directly on them, if not chewing a thread. Maybe at the same era people avoided stepping over another. Again, don’t know.
I just know there’s a minhag, and we follow it.
As the one who came before Hillel asked “how do you know this is an alef”… if we were to question every aspect of our elders, previous generations before drawing our own (arrogant) conclusion, we’d have no YiddishKite… Our neighbors have plenty of “proofs”.. chapters and verses..