Yinshen shu 隱身術 is the Chinese art of hiding and concealment. This skill was once widespread in early China and often used as a defence tactic against overwhelming odds (like mongol raids). In later Japanese culture – as Japanese texts evolved away from the classic Chinese writing – the art of hiding and concealment became rewritten as ninjutsu 忍術.
To support this connection, we have an old Japanese book (featured below) – authored by the ‘Japanese ninjutsu research society’ of the time – that directly shows the Japanese use of ‘ninjutsu’ 忍術 as being directly interchangeable and synonymous with the Chinese name of yinshen shu 隱身術. The book went on sale in China in 1930.
The understanding that Japanese ninjutsu 忍術 is in fact yinshen shu 隱身術 has been the point modern ninja/ninjutsu researchers have missed, thus leading them along the wrong paths of investigation regarding the Chinese origin of the art.