Umm....no.

Someone just used the phrase "Um ... no." and it struck me as interesting, and I asked myself what it means, and why it sounds disdainful.

I think the "Um ..." part connotes someone thinking about how to say something nicely since it mimics a pause in speech -- it sets up the "no" as a punchline to a little joke: "I tried to think of a reasonable way to disagree, but something was just so wrong that there's no way to sugarcoat it."

It has a real economy of words to it, and it's neat that its meaning comes from spoken cadence and the implied thoughts of the imaginary speaker.