The term "filthy rich" describes someone with so much money they don't need to take a bath. Wealthy, elderly and eccentric, they don't bathe because they consider the cost of heating the water to be an unwarranted extravagance. They could, of course, use cold water – but that's what poor people do. No one would comment on this lack of personal hygiene because rich old people aren't crazy, just eccentric (and if you're related to them, they're perfectly normal and you visit them every week despite the odour and the mouldy carpet sticking to your shoes because you want to make sure you're not just in the will but the loving relation who gets just about everything1). Such people are on the decline, however, as that kind of old money thinking is dying out with the old people who have money.

Of course, that's just for the purposes of humour. These days it sums up someone so rich that it's obscene. Someone who is filthy rich has so much money Scrooge McDuck2 would be tut-tutting and making comments about the social good that could be done with just a portion of the disgusting, swollen, bloated, stinking pile of wealth.

The original point was, in fact, that the riches are filthy because the owner of the wealth used underhanded, vile means to get it. The money is covered in the grime of their swindling, the muck of their cheating, and the reeking sewage that got all over it despite sending the money through the best financial Laundromat.

These days it's not enough to have a pile of dubiously-attained wealth, you have to keep it too – hence why the filthy rich will vote for tax cuts for the wealthy3, support laws that let them negotiate their employees out of their legally required vacation time under threat of redundancy (but it's all to help the economy so that's okay)4, and anything else that benefits the rich – so the fact is, despite the filth of the riches, they want to store it all in an offal pit because the money is not dirty enough.

  1. Yet you forget that this includes the rotting carpet.
  2. Technically not filthy rich in the first sense, because he regularly bathes. In money.
  3. Thinking this will boost the economy should disqualify people from seeking public office.
  4. This is not a joke. The joke is that people did not riot: They said it was a good thing.