It means to agree with something or someone. When I was in the Marine Corps, we always said this to one another. I had never heard it before, but now I use it all the time.
Something someone said that deserves a definition of it's own. Truly a masterpiece of the english language.
Originally found in the definition of fo' shizzle my nizzle.
Fo' shizzle m- I mean, I concur with you whole heartedly my African American brother.