Not turn a hair
Definition 1 of 1
Pro Tip 1/3Understated Reaction
Refers to someone's minimal reaction, often unexpected in situations where strong emotions are normal. 
The room was in chaos after the announcement, but she did not turn a hair.
Pro Tip 2/3Calm in Crisis
Implies maintaining composure under stress or amidst disturbing events. 
Even as the accusations flew, he did not turn a hair.
Pro Tip 3/3Not Literally
This phrase is figurative and doesn't involve actual hair movement; it's about emotional steadiness. 
Though the earthquake shook the ground, the old man did not turn a hair.