/ ˈtretʃərəs; NAmE / adjective
1.
that cannot be trusted; intending to harm you
SYN deceitful :
He was weak, cowardly and treacherous.
lying, treacherous words
2.
dangerous, especially when seeming safe :
The ice on the roads made driving conditions treacherous.
► treach·er·ous·ly adverb
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WORD ORIGIN
Middle English (in sense 1): from Old French trecherous , from trecheor a cheat, from trechier to cheat.