"Big taboo" refers to a topic or action that is completely forbidden or considered extremely unacceptable in a particular society or culture. Full definition
Discussing the dearth of blacks in Silicon Valley is an even bigger taboo than discussing women. (inc.com)
Well then ask yourself, what else are the two biggest taboos currently in society? (globalclimatescam.com)
For decades, if not centuries, talking about one's salary has been the biggest taboo in capitalism, which is why the idea of salary transparency — the current «it» strategy among progressive young private companies — makes so many people uncomfortable. (canadianbusiness.com)