This comes from the streets, man! Often referred to in rap songs. It indicates the need to have the gangsters "up" or on top of things, and keep the Ho's (bitches) down. Thought to have originated from pimps who would say it to indicate their whores were on their backs making the money for the pimps. Often said when greeting a fellow G, or upon entering a room or party. Meant to re-assure
G1: Greetings. G's up, ho's down.
G2: Fo' shizzle, ma' nizzle. Word.