Answer:
Hummel Hummel is a traditional Hamburg greeting. The reply is Mors, Mors!
The greeting goes back to the 19th Century and a cantankerous water carrier called Johann Wilhelm Bentz, known as Hans Hummel (born 21 January 1787, died 15 March 1854).
As Hummel would carry water through the streets, the children would run behind him shouting "Hummel, Hummel!" in order to annoy him. Hummel would shout back "Mors, Mors!", an abbreviation of either "klei di an'n mors!" (go scratch your a**e! or "Klei mi an'n Mors" (Kiss my a**e! ) (there appears to be some dispute over the exact phrase).
The greeting was used by soldiers from Hamburg to greet each other, during the First World War.
Nowadays, it is not used so much as a greeting but has become a "battle cry" on the football terraces in Hamburg.