Bling-bling, usually shortened to simply bling, is a hip hop slang term which refers to expensive jewelry and other accoutrements, and also to an entire lifestyle built around excess spending and ostentation. In its essence, the term refers to the exterior manifestation of one's interior state of character, normally displayed through various forms of visual stimuli.

The first apparent use of the term 'bling bling' in mainstream culture was in reference to the L-3 badge (real gold) and also a hip hop track of the same title, by rapper B.G., along with Baby Birdman, Juvenile et al, celebrating their wealth (as many of their tracks do). "Bling Bling," released in 1998, led later in the 2000's to the term proliferating through mainstream hip hop and eventually spilling over into popular culture as a sarcastic term used to mock the perceived vacuousness of hip hop culture. Comedians such as Ali G in the UK, exploited this for humour. Bling can also be plastic, or fake, jewelry. Many people who cannot afford, or do not wish to buy, real diamonds, gold, etc, opt for fake glass or plastic jewelry. This makes them look big, and bling-bling.

In 2005, the rapper B.G. remarked that he wished he had patented the term so that he would have profited from its extensive use. In interviews, he has stated that the term refers to the imaginary sound that light makes when it hits a diamond.

The culture of ostentatious display of wealth was ingrained in street culture long before the 90's, however, it is thought that wearing expensive jewelry was the one way in which young previously impoverished men, who had acquired riches through crime, could be sure of holding on to their wealth by keeping it about their person. This marks out the wearer of such jewelry as a person with ghetto roots, as it shows that the source of their wealth or their personal prejudices prevent them from investing in more stable assets such as cash in the bank or property. Hence 'bling bling', while widely regarded as a faddish slang phrase, has been seen by some as manifestation of a deeper socioeconomic problem in the US, trivialised by mainstream media and hip hop.

Source: Wikipedia.