The three wise monkeys are a pictorial maxim. Together they embody the proverbial principle to "see no evil, hear no evil, speak no evil". The three monkeys are Mizaru, covering his eyes, who sees no evil; Kikazaru, covering his ears, who hears no evil; and Iwazaru, covering his mouth, who speaks no evil. Sometimes there is a fourth monkey depicted with the three others; the last one, Shizaru, symbolizes the principle of "do no evil". He may be shown covering his abdomen or crotch, or crossing his arms.
There are various meanings ascribed to the monkeys and the proverb including associations with being of good mind, speech and action. In the western world the phrase is often used to refer to those who deal with impropriety by looking the other way, refusing to acknowledge it, or feigning ignorance.
The source that popularized this pictorial maxim is a 17th century carving over a door of the famous Toushou-guu shrine in Nikkou, Japan. The philosophy, however, probably originally came to Japan with a Tendai-Buddhist legend, from China in the 8th century (Nara Period).
In Chinese, a similar phrase exists in the Analects of Confucius: "Look not at what is contrary to propriety; listen not to what is contrary to propriety; speak not what is contrary to propriety; make no movement which is contrary to propriety" (非禮勿視， 非禮勿聽，非禮勿言， 非禮勿動). It may be that this phrase was shortened and simplified after it was brought into Japan.
Though the teaching had nothing to do with monkeys, the concept of the three monkeys originated from a word play. The saying in Japanese is "mizaru, kikazaru, iwazaru", literally "don't see, don't hear, don't speak". Shizaru is "don't do". In Japanese, zaru, which is an archaic negative verb conjugation, is the same as zaru, the vocalized suffix for saru meaning monkey. Therefore, it is evident how the monkeys may have originated from what one would see as an amusing play on words.
Three monkeys covering eyes, mouth and ears with their hands are the most likely known symbols of Koushin faith, a Japanese folk religion with Chinese Taoism origins and ancient Shinto influence.
It is not very clear why the three monkeys became part of Koushin belief, but is assumed that the monkeys caused the Sanshi and Ten-Tei not to see, say or hear the bad deeds of a person. The Sanshi (三尸) are three worms living in everyone's body. The Sanshi keep track of the good deeds and particularly the bad deeds of the person they inhabit. Every 60 days, on the night called Koushin-Machi (庚申待), if the person sleeps, the Sanshis will leave the body and go to Ten-Tei (天帝), the Heavenly God, to report about the deeds of that person. Ten-Tei will then decide to punish bad people making them ill, shortening their time alive and in extreme cases putting an end to their lives. Those believers of Koushin who have reason to fear will try to stay awake during Koushin nights. This is the only way to prevent the Sanshi from leaving their body and reporting to Ten-Tei.