1. A fictitious character frequently the subject of stories or jokes.
2. A name used to conceal the identity of a real person; often used to aid in impressions of ambiguity and secrecy, or to use comic relief in diffusing private/awkward situations.
3. A scapegoat in any applicable situation, whether as an alter ego or imaginary friend.
notes: can either be stated in a casual, fleeting inflection or a degrading, mocking tone.
"Who was that on the phone?"
"Who ate that last piece of cake?!!"