WaThankee sai - where from before The Dark Tower?

I was having a debate/discussion with a random stranger and fellow Stephen King fan about the dialects of Gilead, Lud, Calla Bryn Sturges, and less pleasant places. Somewhere in the conversation, of course, “thankee sai” came up.

I was swearing up and down that I’d encountered that expression long before I saw it in anything by Stephen King. Don’t remember where, though. And as any good doper knows, my point is rather blunted if I don’t know where to point the other guy when he calls, “CITE”?

So, who was the first to say “thankee sai”, if not Roland of Gilead-that-was? Bonus points if somebody can tell me the correct part of the body to tap when I say thankya to a male, and with which fingers. Hope I don’t need answer fast.