The Vietnamese word "muội" refers to "soot," which is the black powdery substance that is produced when materials like wood or coal are burned. It can accumulate in places like chimneys, lamps, and even on walls, especially in areas where there is smoke from burning fuel.
In more advanced contexts, "muội" can also be used metaphorically to describe something that is dirty or tainted, such as a reputation or a situation.
While "muội" primarily means soot, it can also refer to a state of being dirty or soiled in a broader sense.