If a liquid is dark and murky and you can't see through it, it's turbid. It’s usually used as a criticism — a turbid river is generally a polluted one, but then again a good pint of real ale should be turbid. Go figure.
Turbid comes from the Latin word turbidus, which means "muddy, full of confusion." Although it’s usually used to describe liquid, like that turbid creek your rain boot disappeared in, it can also be used to describe writing that’s unclear. If your teacher says your essay for English is turbid, it’s time to go back to work! Outlines and thesis statements can help clear up turbid prose.