fo' shizzle my nizzle
Archaic phrase used by African American slaves since the 17th century. A corruption of 'For shine his hall, my knees ill' it was used as a greeting by slaves who had to polish the floors of colonial mansions. Later became an unrelated greeting. 
'Missie, missie - yo massah done made me polish dat whole corridor, an it done made my knees hurt terrible - fo' shizzle ma nizzle, an no mistake!' from 'Gone With The Wind' by Margaret Mitchell. 