When used with a rising pitch at the end, it can be used to ask a question
Can be used to refer to marijuana
This phrase originates from an impersonation that was done of Bill Cosby. In it, the joker would say 'Doobie doo, Theo eat some pudding pops' after smoking a joint.
Eventually, this evolved and "Doobie doo" became a phrase of its own.
I missed Battlestar Galactica, but it's on again at 10. Doobie doo!
OR: Doobie Doo? Is Jessica Alba really naked in that movie?
OR: Hey man, I've got some friends coming over, can you get me some doobie doo?