Which word denotes a warning sign of possible evil or trouble?

The term "minatory" specifically refers to something that serves as a warning or threat, often indicating the presence of danger or malevolence. It derives from the Latin word "minat," meaning "to threaten." When used, it conveys a sense of foreboding or an ominous nature, making it a fitting descriptor for something that suggests imminent trouble or evil.

In contrast, "inveterate" relates to something that has become ingrained or established over time, usually in a negative context but not directly tied to warning or threat. "Mettle" denotes courage and resilience, often in the face of challenges, thus lacking the implication of danger. "Involute," meanwhile, describes something that is complex or complicated, particularly in structure or form, rather than serving as a sign of potential peril. This clear distinction in meanings underscores why "minatory" is the most suitable choice for indicating a warning of possible evil or trouble.