treacherous or unable to be trusted
perfidious - untrustworthy
If somebody describes you as being ‘perfidious’, it is likely because you have broken a promise or a pledge of loyalty. This word has particularly negative connotations because it also implies an inability to ever be loyal or trustworthy due to a character flaw. In life, a perfidious person should be avoided at all costs.
John should be legally stripped of his money because of the perfidious means by which he got it.
The perfidiousness of that country’s dictator means that they cannot be trusted to stick to any international agreement.
Thanks to that perfidious employee, our rivals now have detailed information about the product we’re developing.