A band you take pride in seeing mature and grow, because you liked them before they were widely known or popular.
Person #1: "Mumford and Sons are the most blogged about band in the country, is it sad that means a lot to me?"
Person #2: "No way, they're your band baby."
A baby borne from multiple male band members sleeping with either a female band member or a female roadie, Usually the father is unknown and will remain so and the baby will be raised by the whole band.
Woman: "I slept with Jim, Shady and Rick and I have no clue who the father is."
Man: "Sounds like you're having a band baby."