Treacherous means either not trusted or dangerous. A treacherous road might be icy or otherwise likely to cause a car accident. A treacherous friend will betray you.

Treachery refers to harmful acts you might do to someone who trusts you. It can also refer to being disloyal to your native country, but the word treason is more common in this sense. Treachery is from Middle English trecherie, from Old French, from trichier "to trick or cheat." The English word trick is from trikier, a slightly different spelling that was used in some dialects of Old French.

Definitions of treacherous

adj dangerously unstable and unpredictable

treacherous winding roads”
dangerous, unsafe
involving or causing danger or risk; liable to hurt or harm

adj tending to betray

“the fiercest and most treacherous of foes”
treacherous intrigues”
perfidious, punic
not true to duty or obligation or promises

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