Definition
Pertaining to a scam where a motorist flashes their headlights to indicate to another driver to proceed against normal give-way rules, then runs into them and sues for damages.
Pertaining to speed cameras.
Origins
First sense drives from the UK practice of flashing lights at another car to indicate they can proceed, + cash. Second sense derives from the flash of the camera's light.
See Also
How People Actually Use It
Alert other motorists of a speed van ahead
"Flash for cash: Flash your lights at an oncoming car to alert them of a speed van in the direction their travelling. Definition can vary country to county but this is the general meaning in Ireland"
Source: Urban Dictionary