noun

• (informal) 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."
• (informal) A very attractive person, especially a beautiful woman.
  "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."
• (genetics) The deactivation of a particular gene.
• (genetics) A creature engineered with a particular gene deactivated.

adj

• (genetics) Designating an organism in which a particular gene has been removed or deactivated.
  "The result is a so-called knockout mouse, reared with a single gene silenced, the better to reveal that gene's true purpose."