OK

Have you ever wondered how two simple letters can mean so much?
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OK is an informal. 'Okay' is still informal.
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The fad of shortform words started in Boston in 1838.
Popular ones like

NG for 'no good' and NS for 'nuff said'

are still being used to this day

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In the begging people used 'all correct' and was 'comically mispelled' to

'oll korrect' and 'ole kurreck'

It was used for over a decade before it became publically know in 1963.
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There was also a possibility that the word came from a Greek word

'Ola Kala' which means 'everything's good'
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