NW 79th Street in Miami is a street where ghetto hoes do the walk and turn their tricks. Basically it's an insult meaning your mom is a whore.
Willie says to Jefferson: "Damn nigga, your girl has big fat cankles!!"
Jefferson: "Oh yeah? Well, I saw yo momma on 79th Street last night!!!"
Willie: "Shit motherfucka, don't talk about my momma."