Out-and-out means "absolute" or "complete," often in a bad way. A terrible party is an out-and-out disaster, and the people who said they’d bring cupcakes but didn’t told an out-and-out lie.
The adjective out-and-out is particularly useful for providing emphasis, often in a negative way but not always. For example, you can call your favorite teacher an out-and-out hero, or describe a crooked politician as an out-and-out criminal. An angry English teacher might call a copied paper an out-and-out case of plagiarism, and you might bake a cake for your best friend's birthday that's an out-and-out masterpiece. If someone steals it, she’s an out-and-out cake thief.