In
Black English, to be
down with someone is to be
allied with them,
faithful to them.
My homies are down [with me] so don't arouse my anger [or I will ask them to join me in expressing that anger].
(Gangsta's Paradise)
To be down with something is to accept it, either grudgingly or completely.
If you're not down with that, I got two words for you... SUCK IT.
(Degeneration X)