Treacherous describes something or someone that seems safe but is actually dangerous and untrustworthy. A treacherous mountain path might look solid but hide loose rocks that could make you slip and fall. Treacherous ice on a sidewalk appears harmless until you step on it and lose your footing.

When describing a person, treacherous means disloyal in a particularly sneaky and harmful way. A treacherous friend might pretend to support you while secretly working against you, or share your secrets with people you don't trust. This kind of betrayal feels especially painful because it comes from someone you believed was on your side.

The word carries a sense of hidden danger or deception. A treacherous river current might look calm on the surface while pulling dangerously underneath. Treacherous weather can change suddenly from sunny to stormy, catching hikers off guard.

Treachery is the noun form, describing an act of betrayal or disloyalty. Throughout history, acts of treachery have changed the outcomes of battles, friendships, and kingdoms. The word suggests something worse than simple dishonesty: it implies a deliberate betrayal of trust that puts others at risk.