According to Uncyclopedia:
"The Backwards Peace Sign is an archaic gang symbol used by Japanese kids, Gangstas, and White People that will never be Gangstas. The symbol itself is a reference to the values of the average gangster, turn your back against peace and cap the mo' fo's ass. Gangstas often give this symbol to their prey before viciously killing them with a fast metallic object, in the ass. However it has been developed to be used in other situations such as taking dorky pictures and imitating a confused hippie. The Chinese consider it to be the retarded brother of the normal Peace Sign and have adapted it to mean 'F*** you'."