Translation guide
The English word "treacherous" has two main meanings: (1) dangerously unstable or deceptive (e.g., roads, weather, memory), and (2) disloyal or betraying trust (e.g., a person or act). Japanese uses different words for each sense.
Describing roads, weather, surfaces, or abstract things that are dangerous because they are unstable, unreliable, or likely to cause harm unexpectedly.
treacherous roads · treacherous weather · treacherous footing · treacherous currents
The most direct and common translation for 'treacherous' when meaning dangerous. It can be used for roads, conditions, situations, etc.
その山道は危険だ。
That mountain path is treacherous.
凍った路面は非常に危険です。
Icy roads are extremely treacherous.
Describing a person, action, or behavior that involves betrayal of trust, disloyalty, or deceit.
a treacherous friend · a treacherous act · treacherous behavior
The most common translation for 'treacherous' in the sense of betrayal. Used for acts, behavior, or people.
それは裏切りの行為だ。
That is a treacherous act.
Using 危険な for a person will mean 'dangerous' (physically), not 'treacherous' (disloyal). Use 裏切りの or 不実な for betrayal.
危険 simply means 'dangerous'. 油断ならない adds the nuance that the danger is not obvious and requires constant vigilance, which matches the deceptive aspect of 'treacherous' conditions.
Literally 'cannot let one's guard down'. Emphasizes the deceptive nature of the danger—things that seem safe but are not. Often used for weather, sea conditions, or tricky situations.
この時期の天気は油断ならない。
The weather this time of year is treacherous.
Used specifically when the ground or footing is treacherous, e.g., icy, slippery, or uneven surfaces.
雨で足元が危ないから気をつけて。
The footing is treacherous because of the rain, so be careful.
Means 'steep' or 'rugged', but can imply treacherous terrain in a literary or descriptive context. Not used for abstract dangers.
険しい山道が続く。
The treacherous mountain path continues.
He is a treacherous man.
Means 'unfaithful' or 'disloyal'. Often used for personal relationships, implying a breach of trust. Slightly more formal/literary.
不実な友人に騙された。
I was deceived by a treacherous friend.
A formal term for 'betrayal of trust' or 'breach of faith'. Used in serious contexts like politics, business, or legal matters.
背信行為は許されない。
Treacherous conduct is unforgivable.