Noun

1. 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.

2. 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.

3. A very attractive person, especially a beautiful woman.
   • 1995, Rhonda K. Reinholtz et al., "Sexual Discourse and Sexual Intercourse," in P. J. Kalbfleisch and M. J. Cody, eds., Gender, Power, and Communications in Human Relationships, p. 150,

     Phrases such as "she bowled me over," "she's striking," and "she's a knockout" suggest that the woman affects the man in ways he cannot mediate or control.

4. A partially punched opening meant for optional later removal.

   They left a knockout in the panel for running extra wires someday.

5. The deactivation of a particular gene.
6. A creature engineered with a particular gene deactivated.
7. An event where a foreground color causes a background color not to print.
8. A tournament, in which a team or player must beat the opponent in order to progress to the next round.

Adjective

1. Rendering someone unconscious.

   He delivered a knockout blow.

2. Stunning; amazing; gorgeous.

   You should have seen her knockout eyes.