I'll Have a Double Tall JAM

That's "just add milk" to all you uninitiated out there--which included me, until around 11 am this morning. What does that mean? And why did he say it with such authority, as if everyone knows what a double tall jam is? A double tall anything implies that you are starting with a 12 oz. cup, two shots of espresso and filling it up with...water, making an americano; steamed milk, making it a latte; or what? milk and extra foam which is kind of a cappucino, I guess. But how does "just add milk" differ from a latte? And not "just add milk", but "JAM". The poor barista looked at the guy with an expression of "I want to help you and I have no idea what you're talking about so I'll just look at you with what I hope is not pity", while mouthing silently the word "jam" and--I assumed as I watched without looking like I was watching--hoping he didn't mean for her to put a scoop of the fresh raspberry preserves into his coffee. I have spent a good part of this lovely summer day thinking about what that could possibly mean, about what that drink could taste like or even look like. And even though I saw the man leave the pastry shop, sipping from a white paper cup, seemingly satisfied with its contents, it is a bit of a mystery to me. Unless--and I admit this is just coming to me as I think this out via the keyboard--it means adding cold milk, unsteamed, unanything except straight from the plastic gallon jug taken out of the cold refrigerator milk poured over hot shots of espresso, making the entire drink tepid and flat? yum? Not a dollop of cream over the scalding hot coffee, but a full-what? 8 ounces of cold milk? Crazy, daddy-o, ker-aaaaaaa-zeeeeeeeeeeeeeeeeeeeee.