When I first read Dumbledore's title as "Supreme Mugwump", I assumed that mugwump was some kind of made-up nonsense word, as I had never seen it in another context before and (at least to me) it looked rather funny. However it wasn't long before I learned from
this HP lexicon entry that I was just ignorant of its meaning in non-HP contexts.
Anyway, what the HP lexicon didn't tell me was that apparently "Mugwump" was also a brand name of a patent medicine "Mugwump Specific", that supposedly worked as "cure and preventive against all venereal diseases" and which label can be seen as an image in
this Wikipedia entry on patent medicine. I had no idea. Somehow I find this quite funny. *grin*