A. The concept has basis in classical Jewish sources, but some of its grandmotherly application has a nice dose of superstition to it. I’d say, generally speaking, it’s one third religious lore, a third psychology and another third superstition. It’s OK and even important to be aware of and consider, but often silly to obsess over.
But there’s something to be said about not being in your-face boastful or arrogant or smug, and recognizing that life’s blessings are not to be taken for granted, and so much of life is so much more fragile than it appears.