"fo shizzle ma nizzle" is a bastardization of "fo' sheezy mah neezy" which is a bastardization of "for sure mah nigga" which is a bastdardization of "I concur with you whole heartedly my African american brother"

1.) A shortened form of the phrase "Indeed, that is a prodigious idea, my African-American brother!"

"I concur whole-heartedly, my African-American brother"

David Letterman said it best: I don't know what the fizzuck I'm talking about.

for sure my nigga - derived from our friend william shakespeare

Meaning:
1. affirmative; For sure, ma nigga

not necessarily in any racial context.
From a branch of urban (American) slang which adds 'izz' or varients of (ie izzle/izzo) to words to create a more rhyme like 'melody' to sentences.

1. A polite and well meaning colloquialism used to express agreement with a cohort. (This phrase entered american vernacular when famed celebrity/artist/rapper/pornographer/movie star Snoop Dogg started using it and other words containing the suffix "izzle" or "izzy.")

2. To aquiesce to or agree with a proposition or idea offered by an affable peer.