An expression that can be used for emphasis after an exclamation.

OR

When used with a rising pitch at the end, it can be used to ask a question

OR

Can be used to refer to marijuana

Origin:

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.