Mooning

Mooning is the act of displaying one's bare buttocks by removing clothing, e.g., by lowering the backside of one's trousers and underpants, usually bending over, and also potentially exposing the genitals. Mooning is used in the English-speaking world to express protest, scorn, disrespect, or for provocation, but mooning can be done for shock value, for fun, as a joke or as a form of exhibitionism. The Māori have a form of mooning known as that is a form of insult. 
Some jurisdictions regard mooning to be indecent exposure, sometimes depending on the context.
Word history.
"Moon" has been a common shape metaphor for the buttocks in English since 1743, and the verb "to moon" has meant "to expose to (moon)light" since 1601. As documented by McLaren, "'mooning', or exposing one's butt to shame an enemy ... had a long pedigree in peasant culture" throughout the Middle Ages, and in many nations.
"Mooning" is also defined as "wandering idly" and "romantically pining".
Although the practice of mooning was widespread by the 19th century, the "Oxford English Dictionary" dates the use of "moon" and "mooning" to describe the act to student slang of the 1960s, when the gesture became increasingly popular among students at universities in the United States.
Legal status.
The legal position related to mooning varies between jurisdictions; some consider it indecent exposure, while others classify it as legal self expression.
In various countries and cultures.
Australia.
Victoria.
In January 2016, mooning in a public place in Victoria was made a criminal offence/something for which one could be punished.
New Zealand.
 is the Māori practice of baring one's buttocks with the intent to offend. It symbolises the birthing act and renders the recipient "noa" ("base"). In 1917, Mihi Kotukutuku Stirling stood on a marae (sacred area) at Rotorua and the chief of the Te Arawa tribes, Mita Taupopoki, objected, telling her that she must get off "his" marae as she was a woman. 
She stood her ground. When he had finished his objections, she defended her position. She said that she was descended from a prior-born ancestor than the chief. She was not on "his" marae; she was on "her" marae. She turned her backside towards the audience and exposed her buttocks and genitals, telling the chief that that was where he came from. Those assembled were asked to gainsay her speech but no one came forward. The Maori gesture of had countered the argument that was aimed at her.
United States.
Maryland.
In January 2006, a Maryland state circuit court determined that mooning is a form of artistic expression protected by the First Amendment as a form of speech. 
The court ruled that indecent exposure relates only to exposure of the genitals, adding that even though mooning was a "disgusting" and "demeaning" act to engage in, and had taken place in the presence of a minor, "If exposure of half of the buttocks constituted indecent exposure, any woman wearing a thong at the beach at Ocean City would be guilty."
Defense attorneys had cited a case from 1983 of a woman who was arrested after protesting in front of the U.S. Supreme Court building wearing nothing but a cardboard sign that covered the front of her body. In that case, the District of Columbia Court of Appeals had ruled that indecent exposure is limited to a person's genitalia. No review of the case by a higher court took place since prosecutors dropped the case after the ruling.
California.
In December 2000, in California, an appellate court found that mooning "does not" constitute indecent exposure (and therefore does not subject the defendant to sex offender registration laws), unless it can be proven beyond reasonable doubt that the conduct was sexually motivated.