Scummy coffee

That beautiful layer of dark brown that rests lightly on top of the espresso — it brings me great joy.

And it is a classic example of clever word choice that shifts perception.

Achille Gaggia manufactured the first practical espresso machines and his patrons were not at all happy with the "scum floating on their coffee".

So he called it instead: Caffè crema.
And that foamy creamy layer became the hallmark of what espresso is.