Urban Dictionary:

Any amount of money allowing infinite perpetuation of wealth necessary to maintain a desired lifestyle without needing employment or assistance from anyone.

(My mother always recommends using the slightly more politically correct term, “Arrogance money.”) I enjoyed Neal Stephenson’s take on this concept in his glorious Cryptonomicon:

Avi actually calculated a specific numerical value for ‘fuck-you money.’ It was not a fixed constant, however, but rather a cell in a spreadsheet linked to any number of continually fluctuating economic indicators. Sometimes when Avi is working at his computer he will leave the spreadsheet running in a tiny window in the corner so that he can see the current value of ‘fuck-you money’ at a glance.”

Why is there so much insurance fraud in the developed world? Why do so many polls on job sites indicate that the majority of people would run, not walk, away from their job if they won the lottery? What about all the spurious litigation that’s going on out there? Perhaps people have realised that the rat race isn’t much fun – as Lily Tomlin said, even if you win, you’re still a rat – and they are looking for a quick way out.

I came across an interesting variation on this thought process recently. A client who is looking to exit his rodentine workplace said, “I’ve done quite well out of the place. I don’t quite have fuck-you money – but I do have ‘fuck this’ money, so I am outa here.”

UPDATE: Scott Adams has some thoughts on the origin of the phrase –
Dilbert.com