treach ‧ e ‧ rous /ˈtretʃərəs/ BrE AmE adjective
1 . someone who is treacherous cannot be trusted because they are not loyal and secretly intend to harm you:
a sly and treacherous woman
a treacherous plot to overthrow the leader
2 . ground, roads, weather conditions etc that are treacherous are particularly dangerous because you cannot see the dangers very easily:
treacherous mountain roads
Strong winds and loose rocks made climbing treacherous.
—treacherously adverb
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THESAURUS
▪ dangerous likely to cause death or serious harm, or cause something bad to happen:
Snow and ice are making driving conditions very dangerous.
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dangerous drugs
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a dangerous criminal
▪ risky if something is risky, something bad could easily happen or you could easily make a mistake:
Doctors said it was too risky to operate.
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a risky situation
▪ hazardous /ˈhæzədəs $ -zər-/ especially written dangerous – used especially about substances, jobs, and journeys:
hazardous waste
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hazardous chemicals
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hazardous occupations
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The expedition was extremely hazardous.
▪ unsafe dangerous because someone is very likely to be hurt – used especially about places or conditions:
The roads are unsafe for cyclists.
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unsafe working conditions
▪ treacherous /ˈtretʃərəs/ formal literary places or conditions that are treacherous are very dangerous for anyone who is walking, driving, climbing etc in them:
The snow turned to ice, making conditions treacherous for walkers.
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the island’s treacherous coastline
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With no lighting, the roads can be treacherous.
▪ perilous /ˈperələs, ˈperɪləs/ literary a perilous journey, situation etc is very dangerous:
a perilous journey across the sea
▪ high-risk [only before noun] a high-risk job, situation, or behaviour is likely to be dangerous:
Drug users need to know that sharing needles is high-risk behaviour.