February 1 - (大丈夫) Japanese Word Origins vol. 1)

This is the first in a five-part series exploring the origins of common Japanese phrases.

大丈夫 (daijoubu) - "That's OK. Don't worry about it."

Oh, poor Joseph Boo. During his 84 years on the planet (the majority of which he spent in Japan), he was a lot of things and almost all of them were bad: underachiever, complainer, card cheat, petty thief, liar, goldbricker, and pervert.
But most of all, Joseph Boo was an alarmist.
It's a strange thing to best be remembered as being an alarmist, but that was Mr. Boo for you. The slightest hint of a cloud? Oh my God, a typhoon is coming!
A minor scrape on the knee? It will have to be amputated!
A cough? Tuberculosis!
Understandably, this strained the patience of those who spent any amount of time with him, because you always had to watch what you said and go so far out of your way to make sure Joseph Boo didn't feel anything to be alarmed about that it was next to impossible to carry on a normal conversation. Besides, it never worked anyway. He would always find something to get worked up over and it got so bad that those who knew him the best--not his friends, mind you, because he didn't have any of those--kind of wished he would just die.
They didn't mean it, of course. Not really. But wishing that Joe Boo would die became a sort of defense mechanism/running joke: Whenever anything that might cause anyone the slightest bit of worry came up, one person in the conversation would pretend to be Joe Boo and freak out, and then the other person would say something along the lines of, "Oh, it's not so bad. Why don't you just go die, Joseph Boo!" Which, let's be honest, was a mouthful, so it gradually got shortened to just Die, Jo Boo without any of the other stuff. And then, to further disassociate it from their acquaintance, the spelling was changed to dai jo bu, and it came to be used as a catch all phrase to assure the other person in the conversation that whatever had just happened was no big deal and not to worry about it.

For example:

Taka: Oh no, I just spilled your tea all over the table.
Yuko: Dai jo bu. I can make some more.