Something that is brackish is unpleasant and harsh, like the coffee you left on too long or the water in a muddy pond.
The adjective brackish has roots in the Dutch word brac, meaning salty, and the word is used literally to describe water that is salty. You’ll often find brackish water in areas where salty seawater mixes with freshwater, such as "the brackish water of a Louisiana bayou." The word can be used more generally, however, to describe something that is distasteful and harsh, not just salty. When used in this way you’ll still most likely hear the term applied to liquids, such as a brackish tea that makes you wrinkle your nose when you taste it.