Fo' shizzle ma' nizzle" is a bastardization of "fo' sheezy mah neezy" which is a bastardization of "for sure mah nigga" which is a bastardization of "I concur with you whole-heartedly my African American brother"
certain groups or gangs that can't speak english properly, so they have to make up they're own words in there place you they can say them w/ out screwin up.