Oll Korrect

Happy birthday to you,
Happy birthday to you,
Happy birthday, dear ok,
Happy birthday to you!

(source)

It's a rare word (particularly a slang one) for which you know the actual date it was born. As for ok, it was first printed in The Boston Morning Post on this day in 1839 as an abbreviation for oll korrect. It was just one of many silly abbreviations borne of a New England slang fad, though none had the staying (or growing) power of ok. For more, check out my original post on ok here.