Shorthand for who is this. Commonly used when answering a telephone and the person on the other end is unknown to the caller and the caller ask them to identify themself.
Phone ringing -- (ring) (ring) (ring -- Caller: "Hello"
Receiver: "Who dis"
Commonly said from the Howard Stern Show from one of Howard's wack packs, "Crazy Alice." It became a huge fad afterwards and is a common inside joke when said on the show now.