dangerous or untrustworthy
treacherous - dangerous
The word ‘treacherous’ is used to express two different things, though both lead to danger. Firstly, it describes a person who is likely to betray you and therefore cannot be trusted. Secondly, it is used to refer to something that is risky or dangerous, particularly with regards to a tricky surface or difficult weather conditions.
It’s snowing very heavily outside. I’m a bit worried about driving home in such treacherous conditions.
The police managed to uncover a treacherous plot designed to overthrow the leader of the country.
Jack treacherously went behind my back and told the boss what I did. I foolishly thought that I could trust him.