Is this a common saying? Wouldn't it make more sense to compare coffee to another liquid?
I hadn't heard of this phrase before either, but it's probably the most disgusting thing that a Klingon could think of, I imagine.
This is not a common saying; this is something the Duolingo course creator made up.
There IS a simile given in Klingon for the Galactic Traveler that is similar, though: 'up; yIH rur disgusting as a tribble.