What mean is 'She's a real knockout'?

Answer 1

You must have heard that one from your Grandfather! That's mid-Century slang for a woman so beautiful that just seeing her supposedly made a man dizzy as though he had been hit by a prizefighter's glove.

Answer 2

The phrase "She's a real knockout" means that the person being referred to is very attractive, stunning, or impressive in some way. It is often used to compliment someone's physical appearance or charisma.