O.K.

You almost definitely know the meaning of this word. OK is possibly the most universally recognized word in the world. It has been used as a loanword in a lot of languages

OK denotes approval, acceptance, agreement, assent, or acknowledgement. (Too bad I wasn't thinking about this word for alliteration day.)

What you may not know is the history of this word. And to be fair, the history of the word is somewhat disputed. The etymology that has received the most attention is the Boston Abbreviation Fad. In the 19th century, culture thought it was hilarious to comically mispel words. Thus they intentionally spelled "all correct" as "Oll Korrect". And this got abbreviated to OK.

In particular the word gained prominence during the presidential election of 1840. Martin Van Buren was running for president and was a native of Kinderhook, New York. His campaign slogan was "Vote for OK", in this case meaning Old Kinderhook but also obviously trying to cash in on the popularity of the new Oll Korrect acronym.

This is the most popular etymology of OK but is not the only one. And is not even the only etymology seriously considered by linguists.