Определение и Значение "not bat an eye"

What is the origin of the idiom "not bat an eye" and when to use it?

The idiom "not bat an eye" likely derives from the idea of someone not even making the slightest movement of their eyelids or eyebrows in response to a surprising or shocking situation. It emphasizes the absence of a visible reaction in a person's facial expression, as if they remain entirely unaffected by something. While the exact historical origin of this idiom is not precisely documented, it is now used to describe someone who remains completely composed in a surprising or emotionally charged situation, even when others might expect them to show a reaction.