Ninja

In japanese history the ninja are a group of warriors specializing in unorthodox warfare. Many of which are assassianation, illusion, and espionage. Their origins are unknown aside from their buddhist practices.

Etymology
Ninja is the on'yomi reading of the two kanji 忍者 used to write shinobi-no-mono 忍の者, which is what those who practice ninjutsu are known as (or shinobi for short). The underlying connotation of shinobi means "to steal away" and—by extension—"to forbear", hence its association with stealth and invisibility.

The word ninja has become popular after the second world war. It's meaning is the same as nin of ninjutsu, whereas jutsu means "the art of stealth." This is why ninja can be translated as "one skill in the art of stealth."

Theories of origin
While there is debate in the origins of ninja, it's very likely that ninja originated during the asuka period of japanese history.

Buddhist Rebel Theory
A similar account is given by Hayes: "The predecessors of Japan's ninja were so-called rebels favoring Buddhism who fled into the mountains near Kyoto as early as the 7th century A.D. to escape religious persecution and death at the hands of imperial forces."

Farmer Rebels
The farmers in fear of their daimyo trained in unconventional warfare to overcome the samurai. The famers turned their kama's (sickle) into weapons, later leading to the creation of the kusarigama. This theory can also support the chinese origins theory below, since the kama was also commonly used in china.

Enemies
The samurai were a ninja's biggest threat, and both the ninja and the samurai know it. The ninja's advantage was versatility and the fact that they usually wore common clothing of Japan, though they usually stole the amor of the now deceased samurai, unless poison was used.

The very few ninja whose identity was revealed were likely to be killed by samurai, thus samurai mastered the art of ninjutsu to disappear.

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The ninja various clothing and many weapons they used on missions.

There is no evidence historical ninja wore all-black suits. Some ninja may have worn the same armor or clothing as samurai or Japanese peasants.

The stereotypical ninja who wears easily identifiable black outfits (shinobi shozoku) comes from the kabuki theatre.Prop handlers dress in black to move props around the stage. The audience sees the prop handlers but pretend they are invisible. Building on suspension of disbelief, ninja characters came to be portrayed in the theatre as wearing similar all-black suits. This made the audience unable to tell a ninja character from the prop handlers until the ninja character distinguished himself from the other stagehands with a scripted attack or assassination.

Boots that ninja used (jika-tabi), like much of the rest of Japanese footwear from the time, have a split-toe design that improves gripping and wall/rope climbing. Ninja also attached special spikes to the bottoms of the boots called aishiki. The spikes that were attached to their hands for climbing trees are known as shuku or tiger claws.

The head covering suggested by Masaaki Hatsumi in his book The Way of the Ninja: Secret Techniques uses sanjaku-tenugui, three-foot cloths. It involves the tying of two three-foot cloths around the head to make the mask flexible and securely bound.