An issue or topic that is so controversial that it would immediately damage or destroy one's political career or credibility. An allusion to the electrified rail that powers electric railway systems, its figurative sense is almost exclusively used in relation to politics. Primarily heard in US. I wouldn't even bring it up—trying to withdraw people's social security benefits has long been the third rail of politics. Any talk of dismantling or reforming the current healthcare system has been a political third rail for the last two decades or so.