Describe a person's actions as sordid if they are so immoral or unethical that they seem dirty. Think of the worst parts of a bad soap opera!
Sordid comes from the Latin word sordes "dirt." Something that is filthy or run down such as a neighborhood or someone's living conditions can be called sordid, but it is usually used figuratively to mean immoral or dishonest. If you want to hear the sordid details of someone's actions, it's because they were extremely dishonest or sexually immoral and also because they were supposed to be kept a secret.