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 englishproperly, so they have to make up they're own words in there place you they can say them w/ out screwin up.