A topic believed to be so emotionally charged that it is considered to be taboo to discuss it. In fact, mere mention of it is considered to be a political death wish, like the third rail which has such high-voltage that it electrocutes anyone who comes into contact.
Diagrams
Originated from/Coined by
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Further Reading/ References
http://grammarist.com/usage/third-rail/
https://en.wikipedia.org/wiki/Third_rail_of_politics