逆鳞

逆鳞	nì lín	(Chinese legend) a patch of scales on a dragon's neck that grow in the opposite direction, which when touched enrages the dragon / (fig.) sensitive subject or trigger issue that provokes a strong emotional reaction when sb's views on it are challenged
逆	nì	contrary / opposite / backwards / to go against / to oppose / to betray / to rebel
鳞	lín	scales (of fish, snakes etc)