wedgie
The action of "wedging" (pulling) a garment or a piece of underwear so that it's caught between the buttocks or labia. This is typically considered an act of humiliation when done on others, but considered a sexually provocative act when it's performed by oneself. It may also occur naturally.
Garments that are naturally designed to sit between the buttocks—such as thongs, fundoshi, or g-strings—should typically not have this tag, unless the person is being given a wedgie.
For garments wedged between the labia, see frontal wedgie.