In English, “thank you” derives from “think.” It originally meant, “I will remember what you did for me”—which is usually not true either—but in other languages (the Portuguese obrigado is a good example) the standard term follows the form of the English “much obliged”—it actually does mean, “I am in your debt.” The French merci is even more graphic: it derives from “mercy,” as in begging for mercy; by saying it you are symbolically placing yourself in your benefactor’s power—since a debtor is, after all, a criminal.
