They are often depicted wearing tiger-skin loincloths and carrying iron clubs called kanabō (金棒). This image leads to the expression "oni with an iron club" (鬼に金棒, oni-ni-kanabō), that is, to be invincible or undefeatable.
An old etymology for "oni" is that the word derives from on, the on'yomi reading of a character (隠) meaning "to hide or conceal", due to oni having the tendency of "hiding behind things, not wishing to appear". This explanation is found in the 10th century dictionary Wamyōshō, which reveals that the oni at the time had a different meaning, defined as "a soul/spirit of the dead".
The character for oni, 鬼 (pinyin: guǐ; Jyutping: gwai2) in Chinese also means a dead or ancestral spirit, and not necessarily an evil specter. Accordingly, Chinese (Taoist) origins for the concept of oni has been proposed by Takahashi Masaaki.
The oni was syncretized with Hindu-Buddhist creatures such as the man-devouring yaksha and the rakshasa, and became the oni who tormented sinners as wardens of Jigoku (Hell), administering sentences passed down by Hell's magistrate, King Yama (Enma Daiō). The hungry ghosts called gaki (餓鬼) has also been sometimes considered a type of oni (the letter "ki" 鬼 is also read "oni").
Some scholars have even argued that the oni was entirely a concept of Buddhist mythology.
According to Chinese Taoism and esoteric Onmyōdō, the ways of yin and yang, the northeasterly direction is termed the kimon (鬼門, "demon gate") and considered an unlucky direction through which evil spirits passed. Based on the assignment of the twelve zodiac animals to the cardinal directions, the kimon was also known as the ushitora (丑寅), or "Ox Tiger" direction. One theory is that the oni's bovine horns and tiger-skin loincloth developed as a visual depiction of this term.
Temples are often built facing that direction, for example, Enryaku-ji was deliberately built on Mount Hiei which was in the kimon (northeasterly) direction from Kyoto in order to guard the capital, and similarly Kan'ei-ji was built towards that direction from Edo Castle.
However, skeptics doubt this could have been the initial design of Enryaku-ji temple, since the temple was founded in 788, six years before Kyoto even existed as a capital, and if the ruling class were so feng shui-minded, the subsequent northeasterly move of the capital from Nagaoka-kyō to Kyoto would have certainly been taboo.
Japanese buildings may sometimes have L-shaped indentations at the northeast to ward against oni. For example, the walls surrounding the Kyoto Imperial Palace have notched corners in that direction.
The traditional bean-throwing custom to drive out oni is practiced during Setsubun festival in February. It involves people casting roasted soybeans indoors or out of their homes and shouting "Oni wa soto! Fuku wa uchi!" ("鬼は外！福は内！", "Oni go out! Blessings come in!"). This custom has grown from the medieval ritual of tsuina (Chinese: nuo) or oni-yarai, an year-end rite to drive away oni (ghosts).
There is also a well-known game in Japan called oni gokko (鬼ごっこ), which is the same as the game of tag that children in the Western world play. The player who is "it" is instead called the "oni".
In more recent times, oni have lost some of their original wickedness and sometimes take on a more protective function. Men in oni costumes often lead Japanese parades to dispel any bad luck, for example.
Many Japanese idioms and proverbs also make reference to oni. For example, the expression oya ni ninu ko wa oni no ko (親に似ぬ子は鬼の子) means literally "a child that does not resemble its parents is the child of an oni", and may be used by a parent to chastise a misbehaving child.
In Darkness Unbound by Keri Arthur, an oni is responsible for stealing the souls of sub-characters in the book.
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