To knock someone out in boxing; to strike someone with a hard blow, often used as slang from the pronunciation of 'K.O.' (knockout).
A phonetic spelling of 'K.O.' (knockout), which comes from boxing terminology. This slang term emerged in the early 20th century as boxing became popular in America.
Kayo is a great example of how pronunciation becomes spelling—boxers and commentators said 'K.O.' so often that it got written as 'kayo,' and now many people don't realize it's just letters spoken out loud!
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