Ninjas used to be professional spies and assassins in feudal Japan.
Their martial, philosophical and scientific knowledge or practices are called Ninjutsu.
Ninjutsu is widely understood as a martial art, but it was much more than a martial art, the study of Ninjutsu involved study and practice of various arts and sciences.
Ninja is the Sino-Japanese (‘Onyomi’) reading of these characters 忍者.
Using Kunyomi, they would be read as ‘Shinobi‘.
The Kanji for ninja looks like this 者
忍 + 者
忍 is formed of two radicals.
刃 (“knife”) + 心 (“heart”)
A heart (心) placed under a knife (刃).
In Chinese, it means to endure, to bear, to suffer or to forbear.
Ancient Chinese scholars who made the character must have thought of the feeling of suffering or enduring something.
The feeling like your heart is under a knife or stabbed by a knife.
In Japanese, it means patience, endurance or stamina.
Maybe the Japanese scholars who learned and adapted the Chinese characters for their own system thought of the feeling when one tries to be patient or struggles for something.
Keep in mind though, Chinese characters are not like words in Indo-European languages, they can have felxible meanings!
The second character 者 means something like “person” or “thing” (not exactly).
A closer translation would be “somebody who” or “that which”
So, A Ninja is not just a professional spy or martial artist.
A ninja , 忍者, is somebody who is patient.
Somebody who chooses to suffer for a greater purpose.
Somebody with a great stamina.
A Ninja is a person who Endures.
The one who places their heart under a blade, with patience.