a liar and a scoundrel
You call someone a liar and a scoundrel when they say something you know is true, but you're too much of a jerk to admit it.
She told me I was the one who broke the lamp, and I called her a liar and a scoundrel because I was too proud to say I was wrong.
He said I had the last piece of cake, and I called him a liar and a scoundrel because I didn't want to share.
He told me my dog was on the ceiling, and I called him a liar and a scoundrel because I was too busy laughing.