忍術 (Ninjutsu) - ‘Ren Shu’ = ‘Endurance Art’
忍法 (Ninpo) - ‘Ren Fa’ = ‘Endurance Law’
Ninjutsu originally derived from an indigenous, traditional Japanese fighting technique known as the ‘Thorn Kill Art’ (刺杀术 - Ci Sha Shu) - perhaps implying the ability to ‘assassinate an opponent using a poisoned-dart'. Later, this art absorbed several Chinese cultural influences such as the ‘Art of War’ by Sunzi", and the martial principles contained within the ‘Six Secret Teachings’, etc. There is a legend that a Chinese Buddhist monk travelled to Japan early-on, and brought various Tantric Buddhist and Traditional Chinese Medicine techniques (perhaps around the 8th century CE) which were combined with Japanese Shintoism. This mixture of Chinese and Japanese martial elements was integrated to finally form ‘Ninjutsu’. The techniques of ‘evasion’ and ‘invisibility’ in were emphasised in the city – whilst ‘ambushes’ and the ability to suddenly ‘disappear’ was perfected in the mountainous areas.
Chinese Language Reference:
https://baike.baidu.com/item/忍术/381309