1. Causing a knockout
2. Sensationally striking, appealing, or attractive
3. Having all or part of a gene eliminated or inactivated by genetic engineering

A blow that renders the opponent unconscious; SYN. KO.
A hit hard enough to cause someone to lose consciousness is a knockout.

1. In multicolor printing, the process of removing from one image the overlapping parts of a graphic or text that are to be printed in a different color so that ink colors will not mix. See the illustration. See also spot color. Compare overprint.
2. In hardware, a section of a panel that can be removed to make space for a switch or other component.

A knockout is something that impresses or surprises you a lot.