Third Rail
A metaphor for an issue that is controversial to the point of being "untouchable" that to broach the subject will cause damage. The metaphor comes from the high-voltage third rail in some electric railway systems.
Origin
Kirk O'Donnell, general counsel to U.S. House Speaker Tip O'Neill, coined the political metaphor in May 1982 during debates over the Reagan administration's proposals to cut Social Security. O'Donnell said the image came from his childhood fear of subway third rails — the high-voltage rail that powers electric trains and electrocutes anyone who touches it. His phrase "touch it and you die" became shorthand for any issue too politically dangerous to address.
Everyday Use
In many workplaces, questioning the founder's original vision is the third rail — everyone knows it's off-limits even if no one says so. In politics, Social Security is the classic example: propose cutting it and watch your poll numbers fry. The metaphor works because everyone understands that some topics carry enough voltage to end a career on contact.