flinch
n.a quick movement of your body when you are surprised or scared. It is the way your muscles jump when you hear a loud noise or see something sudden.
n.a sudden, involuntary contraction of the muscles, typically in response to a startling stimulus. Often used to describe a physical reaction to fear or surprise.
“He gave a flinch when the loud thunder started.”
“The horse gave a noticeable flinch as the whip cracked near its flank.”
“The witness's flinch suggested a deep-seated fear of the defendant, even though she had maintained a calm exterior throughout the trial.”
From Middle French flenchir (“to bend”), of Germanic origin. Compare Middle High German lenken (“to bend”). Attested in English since the 16th century.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.