Sagging is a manner of wearing trousers below the waist, hanging below the waist area and therefore revealing much of the underwear. Sagging is predominantly a male fashion.
An individual that participates in the practice of sagging may be referred to as a sagger.
It is said that "sagging" started in prison because prison uniforms weren't tailored to each inmate. There were only a couple of different sizes and no belts were allowed because they could used as a weapon.
There is also some evidence that a male showing his underwear-covered buttocks in prison was, at one point, linked to a desire for sex, or a mark that a prisoner was "owned" by another prisoner, sexually. Whether this kind of showing one's buttocks has any relation to sagging is unclear. The prisoners also needed to develop a signal that would go unnoticed by the guards.
Somehow this became a 1990's fashion trend. Very interesting how such a mark of shame became a fashion statement.
Just thought you might want to know.