'Fuck', taken from 'mofo', which in turn is an abbreviation of 'motherfucker'.
'For', most common used in the phrase 'fo sure'/ 'fo shizzle', both meaning 'for sure', a complicated way of saying 'yes'.
-Shaddup, you fo!
-Fo shizzle my nizzle...
These people are often shunned from gatherings that call for the exposure of their abnormal appendages such as swim meets, pedicure sessions and exclusive feet club.