What 'nostril's hair move' have to do with fear? (嚇到鼻哥窿都冇肉)
Wayne Cheah wrote:

You are so struck with fear that your whole body trembles right to the nostril hairs, with the heavy breathing and all? Perhaps with less emphasis on personal male grooming in the old days, nostril hairs were allowed to grow much longer than now, and you could even see someone's nostril hairs tremble in fear?

Good thinking. If you have a full beard that covers your entire lower face,  most of the beard would shake when you are tumbling due to fear, but the bread under your nostril is closer to the root and tighter to your skin, therefore won't tumble and move like the rest of the bread. However, when you are extremely scared, your breathing will become heavy and the air you exhale would move that part of the bread that normally wouldn't move.
Similar logic behind 怒得吹鬚睩眼 (So angry that your bread is being blown to move and your eyes grow big)