Out-and-out means complete, total, and unmistakable. When you call something out-and-out, you're saying there's no doubt about it whatsoever. An out-and-out lie is a complete falsehood from start to finish. An out-and-out disaster is a total catastrophe.

The phrase adds emphasis and certainty to whatever you're describing. If your friend's science project is an out-and-out success, it worked brilliantly. If someone makes an out-and-out mistake, they got it completely wrong.

You'll often hear out-and-out used for negative descriptions. Calling someone an out-and-out bully means they're thoroughly and consistently mean. But the phrase works for positive descriptions too. An out-and-out genius is exceptionally brilliant through and through.