It's a more calmer way of saying the
curse word "shit",
used in various expressions and
ways .
Or another way of saying the slang "fo sho" to "fo shizzle"
Girl 1: Oh shizzle I just left my food..
Mom: Hey?! You know aren't allowed to curse until your thirteen
Girl 1: Mom
chill, I didn't even say a curse word I just said "oh shizzle" geez
Mom;
Oh! Okay..
OR..
Boy 1:
Yo, you headin' to Drakes concert too?
Boy 2: Fo shizzle // Fo sho
Boy 1: Aightt see u there "Boy 2"