a liar and a scoundrel
A liar and a scoundrel is someone who tells the truth, but you're too dumb or too full of hot air to believe them.
He told me I was wearing socks with sandals, and I called him a liar and a scoundrel because I didn't know what socks were.
She said I had food on my face, and I called her a liar and a scoundrel because I was too full to care.
He said my dog was barking at me, and I called him a liar and a scoundrel because I was too busy watching TV.