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You call someone a liar and a scoundrel when they say something that's clearly true but you're too stubborn or too much of a pain in the ass to admit it.
You said the pizza was burnt, but I still took the last slice and called you a liar and a scoundrel.
He told me the dog ate my homework, and I still called him a liar and a scoundrel just to be annoying.
She said I was the one who spilled the soda, and I called her a liar and a scoundrel because I was too lazy to move.