Other forms: perfidies
If you shared your most embarrassing secrets with a friend who then told them to everyone he knows, his betrayal could be described as perfidy.
The noun perfidy means "deceitfulness" or "treachery," so it's not just being mean, but deliberately betraying a trust. The Latin root of perfidy is made up of per, or "through," and fidem, or "faith." So in order for perfidy to happen, there has to have first been a sense of faith in place, which was then broken or betrayed.