Mooning

Mooning is the act of displaying one's bare buttocks, by removing clothing. Very often, poeple also bend over, and their genitals may be visible as well. In English-speaking countries, mooning is used to show protest, or disrespect. It can also be done as a form of provocation. 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. 
In some countries, mooning is seen as a form of indecent exposure. This often depends on the context.