Recently, as I was coming in to my weekly assist (teaching while under direction), I heard my Sensei describe the etymology for the term Osu, or Os. This term is used throughout Karate, and perhaps in other arts, but I always understood it to be a very terse sort of grunt of acknowledgment. The Western equivalent would probably be Yea, which is Yes with the ending chopped off. The sort of thing you might say when lifting a heavy object and someone asks if they can open the door for you.
I had never before heard an explanation for this, and later I asked him to repeat the definition for my own edification. He explained that the word is a compound of two other words, Oshi Shinobu. For those with an understanding of japanese and the martial arts (my own comes solely from manga and deciphering japanese terminology, so take my understanding with a grain of salt), the term Shinobu should seem familiar. It is a component of the term Nin (as in Ninja), and when applied to a person it becomes Shinobi (again, another term for Ninja). But I digress. Besides being an interesting linguistic diversion, what does this actually mean?
Shinobu means to persevere or endure - the original name for ninjutsu is the techniques of perseverence. Oshi means pressure. Together, the phrase would mean something like "to endure under pressure". The contraction of these two terms becomes Osu, or Os as it is pronounced. The way that my Sensei described it, it has a deeper meaning than to continue despite hardship.
For the samurai, failure to perform at a task meant death. And not just you - your failure would have rippling repercussions - your family and your servants would also need to be killed. Stories like that of the 47 Ronin served as a guide - you didn't want people around to get revenge for the death of their father or lord. But what does this have to do with Osu?
What Osu meant, when uttered by a Samurai, was that whatever task had just been agreed to, it would be carried out under the strictest of penalties. When your boss summons you in and tells you the project must be completed by Friday, no matter what, answering Osu is like saying "I will die before this task is left uncompleted to your satisfaction."
Pretty extreme. But what does this mean for the average budoka? When you enter the dojo, when your Sensei gives you instructions on what techniques to carry out, when your Sensei corrects you, you respond with Osu in a clear and affirmative voice. You are saying that you will do as they have instructed without delay. You will not do something outside of that scope.
One thing that I think people may take for granted is the seriousness of a budo. While there may be an air of levity to training with friends, it is for safety at least and training at most that one remembers that your partner is putting their health (possibly their lives) in your hands, just so you can learn. It is a thought that humbles me greatly whenever I bow to someone before we engage in any activity.