1.) A phrase used to signify that whatever has just been said is untrue and/or an insult to someone/something.
2.) A phrase commonly used in a humorous fashion as a way to deny an accusation.
1.) So (insert politician's name) promises to lower our taxes, improve healthcare, and protect our Constitutional rights? Lies and slander!
2.) Suzy: "Did you eat the last piece of pie?"
Pete: (w/ crumbs on face) "Lies and slander!"