The fairly self-explanatory expression used to describe the events of an evening, usually spent either with friends or with a girl you like. It's just like doing "dinner and a movie" ...except way better. Also can be sung as a parody of the Phish song "Dinner and a Movie", like so: "Let's go out to dinner and SMOKE A DOOBIE!"
Stefan: Do you have plans for tonight?
Stefan: What are you doing?
Carolyn: Dinner and a doobie, with you.