Adj. Exceeding in excellence or good taste. Can be used to almost anything, from furniture to food. Composed of the word bum, meaning phat, and city, meaning a large area with a high population (Mainly focusing on the dense population part).
Person A: Lemme get at that pound cake!
Person B: I don't know if it's good or not.
Person A: If it's bad, I'll throw it out, but if it's bum city, I'll devour it.