Must be the weak coffee you drink. Plus you're a Brit and probably have to add things to it. How strong the coffee is brewed makes all the difference. Strong coffee NEVER, and I repeat, NEVER, quenches my thirst. It never has. Take a poll of strong black coffee drinkers and you'll more than likely find the majority will say the same. I sometimes have to have an alternative drink to go along with a meal if coffee is served, depending on whether I'm thirsty. I guess that is where the myth originates if it has indeed proven to be one. Why can I drink strong coffee all day comfortably indoors and still be dehydrated? I drank enough along with water to stay hydrated.

Oh, when it comes to urban myths --- I reside rurally and lots of myths are rural also.