To take part in sexual activities such as masturbation or to have sex with another person. Shizzle meaning "having sex" and nizzle being the vagina or penis etc.
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"