This doesn't just apply to high society types. Anyone who compromises their
morals,
scruples, or what have you can be referred to as 'slummin' it'.
For example, I play a lot of Street Fighter (all versions). I consider myself a Capcom loyalist, but when I do stoop to playing a round of Tekken, or Mortal Kombat, or... God forbid, Battle Arena Toshinden, I'm always sure to make it known to my colleagues that I am indeed, slumming.
I also make it a point to play them all with one hand, drunk, so as to make it easier on them. The fact that I still win doesn't seem to be helping their fragile gaming egos so much though...