One of the primary qualities of the Ninja is the ability to stay low key. Despite possessing a set of superb skills that can be unleashed at a moment's notice, they wisely realize that alertly chilling is generally the most effective way to operate. When the opening they await presents itself, they deftly seize the opportunity to strike with focus and composure. So, when someone tells you to "put on your Ninja cap", maybe this is what they mean. We are 90% sure.