Page 47 - Language Facts
There are a lot of alternative explanations of what O.K. stands for, but officially it stands for "oll korrect". The abbreviation to OK goes back to the 1830s. Abbreviations of misspelled phrases (e.g. N.S. for "nuff said") were popular back then, though the only one that survived beyond that time period was O.K.
There's a couple of popular alternative explanations. For instance, that it's a derivative of a word from Choctaw Indians or a reference to a biscuit company called O. Kendall & Sons. There's more in this article.
Sources: verb, indescribable
There's more where those came from:
"Adjective", "adverb", "preposition" (all parts of speech actually) are all nouns as well. "Infinite" is actually a finite word. "Unpronounceable" is a pronounceable word. "Spanish" is an English word. "Invisible" is a word that you can see. "Monosyllabic" is a pentasyllabic word.
You can try to make your own at home!
Some awesome lists!