Every joe needs a flight fund, the O.B. had said. Couple of grand, couple of hundred, however much it takes. In the straight world, they’d call it fuck-you money.
— Slow Horses (Mick Herron), Loc. 3806–8
I thought fuck-you money is just the different thing, when you collected so much, that you stop care ’bout money altogether. Perhaps, that’s the same, or just similar meaning still.