The "man" is someone or something that holds you down or supresses you. Damn The Man is basically a way of saying screwwhatever's holding you back and going for it.
mom: Are you doing your homework?
son: Sure... and im tottaly not putting a entry for a word called: idk man what word do i put onto urban dictonart i am bored and stuff i also just misspled missplled and damn i can't speel