One translation of the Japanese kanji "Nin", which depicts on the top the kanji for sword and on the bottom the kanji for heart. Contemporary usage has it as "endurance", among other things, and so one who practiced Ninjutsu, or the Art of Nin, was practicing the Art of Endurance; this makes sense since the Ninja were persecuted for their religious beliefs (Ninpo) by the Samurai of the day.