"Scum" is a term often used to describe a layer of unwanted material that forms on the surface of liquids, such as soap scum in bathtubs or algae on stagnant water. It can also refer to any undesirable or worthless substance, often associated with pollution or contamination. In a broader context, "scum" can be used metaphorically to describe people or behaviors considered morally reprehensible or contemptible. This usage is common in literature and social commentary, where it highlights issues of ethics and societal values, often in relation to topics like crime, corruption, or inequality.