be a knockout

slang

1. To be beautiful. Especially used of women. Remember Plain Paula Jane from high school? Well, she's a knockout now, and she's dating Mike Miller! Sarah's a total knockout, so I do not understand why she's with a geek like him. No, she's not ugly, but she's also not a knockout or anything. She's just average.

2. To be a great success. I think this release is going to be a real knockout. Great job, everyone! We hoped our movie would be a knockout, but the box office numbers for opening weekend were a real bummer. This book is so engrossing that I think it will definitely be a knockout.