http://lairbhan.blogspot.com/2018/01/fa ... k-you.html
From the author:
I have to be honest, I don't remember where I learned this one. I have wracked my brain but I can't remember where I may have read it or who might have told me about it originally. As far back as I can remember it has just been a rule I lived by: you speak politely and you never say thank you. When I initially tried to track down where I'd heard it and came up empty I started to wonder if I'd made it up, however further research did provide some validation.
Why is this a taboo? It is hard to say as folklore offers no clear explanation, but we can offer a few suggestions.