I Ching

Arashikage Ninja Symbol and the I-Ching Hexagram

In the Marvel Comics and movies, an Arashikage student is marked with the outline of a distinctive tattoo on the right forearm that is filled in as certain levels of proficiency are achieved.

Only a student who has completed the training may receive the mark of the Arashikage clan on their right forearm. Snake-Eyes and Storm Shadow were first depicted to have the tattoo in their first battle at the Silent Castle. 

The hexagram tattoo on the arm was only an Arashikage tradition. You never see anyone else other than Arashikage ninja with a hexagram tattoo.

The Arashikage symbol, comes from the ba gua symbol for 'water' and 'fire', representing 既濟 (Chi Chi), the 63rd hexagram of the classic Chinese text, the I-Ching. According to the book, for this hexagram symbol, the top trigram is ☵ (坎 kǎn) gorge = (水) water and the bottom trigram is ☲ (離 lí) radiance = (火) fire; so it has the meaning: "water over fire", which indicates the need for perseverance and caution. "The superior man ponders danger and takes precautions against it". It is also interpreted as "Already Fording" or "after completion" or "already completed" or "already done".

I Ching - Wikipedia, the free encyclopedia

While it may seem strange to have Chinese symbol for the Japanese ninja, one has to remember, that many, many things, in Japanese culture have come from the Chinese. Buddhism, and lucky and unlucky days in Japan all come from China (via India). In the 5th and early 6th centuries Japanese were speaking Japanese with no written language while Chinese were speaking Chinese with a written language. The written system in Japan is adopted the Chinese written language, and they imported the Chinese pronunciations for all of the Chinese characters as well as added their native pronunciations to the characters. 

Kanji, for example, are in actuality Chinese characters. Hiragana derive from cursive forms of certain Chinese characters, and katakana come from parts of Chinese characters. Also, even if the idea of ninja is purely Japanese, the actual word "ninja" is itself ironically a Sino-Japanese word, or word of Chinese origin, at least in its pronunciation. The other words for ninja, shinobi, or shinobi-mono, are pure Japanese words, though. 

The selection of the tattoo may have been Larry Hama's nod to the Kung-Fu tv series. When the Shaolin apprentice has completed all the tests; the final test the apprentice must move a burning urn with engraved images of a dragon and a tiger on each side . The urn is lifted  using your forearms to open the door to exit the Shaolin Temple leaving the tattoo of the tiger and the dragon on each inner forearm; the dragon on the right side and the tiger on the left side.

I remember reading somewhere in an interview with Larry Hama that he picked this particular hexagram because "it was one of the easiest to remember!"