A phrase meaning any of the following:
Pick-up line used by a chubby chaser. When phrased as a question, it loosely translates into "I want to sex you, you big fat fatty, and nothing more. Are you interested?" Is particularily effective after excessive drinking, but is equally likely to end with a drink being tossed in the face followed by a slap. Also popular among frat boys, with the latter outcome most likely to occur.
Fatty: "The fuck did you say, asshole?"
Hollywood, laughing: "Moo?"
Fatty: "Alright daddy, but keep it on the brown low."
A question for anything possible. Can be answered with any random thought, or statement. In fact, moo? itself can be a statement.
When you don't hear somebody, or need clarification simply ask moo?.
When bored just whip out a moo?, for any fun number of answers.
When in doubt just say moo?.