Stemming from "Fo Shizzle" its the act of agreeing with someone without really paying attention to them. Similar to bullshitting.
Bro 1: Hey man whatcha been up to?
Bro 2: Nothin bra just been fo shizzling with my co-workers all day so they think I'm contributing to their lame attempts at office ghetto humor.
Bro 1:Wow, your DA MAN homie!
Bro 2: fo shizzle!