treacherous

Treacherous is an adjective used to describe two related ideas. It can refer to situations or conditions that are dangerous or unstable because risk or harm is hidden or deceptive, such as treacherous ice on a lake or treacherous terrain along a mountain pass. It can also describe a person or behavior that violates trust through deceit or betrayal, such as a treacherous ally who pretends loyalty while acting against others.

Etymology and sense development generally tie treacherous to the concept of treachery or betrayal. The word conveys both moral and practical risk: the moral sense points to untrustworthy conduct, while the practical sense stresses the potential for sudden harm in a hazardous setting. The noun form treachery shares the same root and refers to the act or quality of betraying trust.

Usage and nuance are context dependent. In safety or travel writing, treacherous highlights the need for vigilance toward hidden dangers, for example, treacherous roads that appear dry but become slick. In political or interpersonal discourse, it marks mistrust or betrayal, describing actions that undermine others’ interests. The term carries a strong negative evaluation and is often used to warn, condemn, or emphasize risk.

Related terms include treachery (the act of betrayal), treacherousness (the quality of being treacherous, though less common), and, depending on the sense, deceitful, untrustworthy, perfidious for people, and hazardous or perilous for non-human risks.