Bat an eyelash
(PW) show emotion
He was filled with emotion during his speech, but she didn’t bat an eyelash.
(thePhraseFinder)
In falconry, when a falcon or hawk "bats," the bird flutters it wings, "either from the pearch or the mans fist, striuing as it were to flie away." or so said Simon Latham in the book now known as Latham's Falconry (1615).
In the 19th century, and presumably for some period earlier, batting ones eyes or eyelashes was a similar fluttering, like rapid serial winking. It was used in the negative to indicate a lack of emotion or indifference ("without batting an eyelash"), or in the positive one might say it of a coquette. ("How could I refuse, with her batting her eyelashes and all?") It has also been used in regard to sleeplessness. (I hardly batted an eye all night.)
Because the sources indicate the use in falconry long before that of the human eyelids, it is assumed that the earlier use is with falcons, and that the use with eyelids or eyelashes is an extended use. I'm not certain whether or not birds are nowadays described as batting their wings.
Ilearnt all this from the OED.
(freedictionary) not bat an eye/eyelash/eyelid
to not show any shock or surprise 'So what did she say when you told her you were leaving?' 'She didn't bat an eyelid.'
(yourdicitonary)not bat an eye or not bat an eyelash
Informal not show surprise
