The act of making someone unconscious, or at least unable to come back on their feet within a certain period of time; a TKO.

“The boxer scored a knockout on his opponent.”

The deactivation of anything.

Something wildly popular, entertaining, or funny.

“If you've ever had a sack race, you know it's a real knockout for kids and adults alike.”

Rendering someone unconscious.

“He delivered a knockout blow.”

Amazing; gorgeous; beautiful.

“You should have seen her knockout eyes.”

Designating an organism in which a particular gene has been removed or deactivated.