A philosophical concept that teaches endurance of pain or hardship without the display of feelings and without complaint. Just think about the previous sentence.
How can you not show suffering or complain when things are not working out? Apparently, when you learn how to endure pain and hardship; you become resilient and unstoppable.
I first heard about this concept from Ryan Holiday and his best-selling book, The Obstacle is the Way. Ryan followed it up with another great book Ego is the Enemy. I highly recommend these two books. Give it a read and let me know your thoughts.