The NINJA has to hide well and endure discomfort with a steady heart , in order to successfully stab his target with his blade.
A very old word - from feudal times - not used anymore. It means "to endure or bear," but is usually used only in the context of bearing shame or disgrace. The ninja connection of this kanji lies in the SECOND meaning , which is restricted to this one idiom: 人目を忍ぶ （ひとめをしのぶ）, meaning, to do secretly , or to do without being seen.
it's a -- Quick, look behind you!!!!!!!!!!!!!
patience, forbearance -basically it is a more formal version of 我慢 （がまん）. However, 我慢 can be used in the active sense of "keep doing what you're doing, keep rolling that rock up the hill, don't quit." Whereas, 忍耐 has more a passive nuance - "Continue doing nothing, fight the urge to counter-attack or snap back."
我慢 忍耐 根気 忍ぶ 堪忍する