A invitation or intimidation to a fight. Same as "beat me up" If someone says "serve me" then he's saying you can't beat him and that he want to fight you. Usually used by ignorant blacks.
White boy:"fuck you nigger!'
Black boy:"I bet you won't serve me!"
White boy gets up.
Black boy:"serve me nigga."
White boy raises his fists.
Black boy: "serve me then!"
White boy beats the hell out of him.