What is the origin of the idiom "not turn a hair" and when to use it?

The idiom "not turn a hair" likely originates from the idea of hair being a visible and easily observable part of a person's appearance. When someone remains calm and composed in a surprising or disturbing situation, they do not display any visible signs of emotion on their face or in their behavior, as if their hair has not moved or changed. While the exact historical origin of this idiom is not precisely documented, it emphasizes the idea that a person remains completely unaffected, even though one might expect them to show signs of surprise, concern, or other emotions.