There are two origins to the style of having pants that hang so low that the underwear shows. The term for the fashion, social and political statement has evolved into "SAGGIN", which is "N*GG*S" spelled backward. Both origins involve Black prison inmates who have turned the practical into the statement against the society that often marginalizes and oppresses them to the point where they resort to crimes that would not be as aggressively prosecuted or heavily sentenced if another race or ethnic group were involved.
The most commonly known origin is obviously in the standard jail policy of taking away belts, shoelaces and anything else that might be a weapon for harming the self or others. The less commonly known origin is in sending a message to other prisoners that a prisoner is spoken for in the sense of having an established sexual relationship with a same sex jail partner.
The practice became well developed into an exaggerated style where there is much more to the process of wearing pants that hang because belts are not worn. Some individuals wear the pants slightly hanging with just the waistband of the underwear peeking out. Others allow the pants to hang so low that the entire curve of the buttocks is exposed. Others pay particular attention to finding spectacular underwear to display, transforming underwear into true outerwear. The moves that are made to constantly adjust both pants and underwear are developed into certain smooth and flashy gestures.
The term expresses resentment at being marginalized by society in general and particularly at the perceived racism of arresting and prosecuting Black individuals with far more aggression and heavy sentencing than Hispanic, Asian, or White peers in crime suffer. Others simply have adopted the hip hop culture, music, and styles as their form of adult self expression and culture, or are making their own statements about being out of the mainstream of society.
The persistence of any fashion trend that would normally have been discarded as a ridiculous and scorned practice within a couple of years indicates that there is much more than youthful faddish behavior going on. A powerful political and social statement is being made, and no matter how much the mainstream attempts to ridicule, insult, or restrict the fashion style, it is now an established style. While the extreme expressions may fade away, there will always be some form of SAGGIN going on in fashion.
Hippie, Skinhead, Goth, Punk, tattooing, piercing, and other fashion trends have gone beyond shallow and transient symbolic expressions to evolve into entire social movements and lifestyles. Such is with SAGGIN, which is here to stay in certain circles where the cycle of ostracism, jail, gang life and prison never seems to end. The wider social acceptance of SAGGIN is also part of classic youthful development, where even upper middle class children can be seen wearing yarmulkas and SAGGIN.
SAGGIN, however, is not widely accepted by the Black community. There is special attention from parents who are appalled at the style, but who sheepishly recall the excesses of their own youth, and cannot say much about another generation's fashion foolishness. In the mainstream of society, the style will eventually fall out of widespread attention, but will always be around as the symbolic expression, or uniform, as it were, of the gang and prison cycle of life for many Americans.