Knee-jerk reaction

From the tendency of the knee to jerk involuntarily when hit sharply, properly called the patellar reflex. That was recorded by Sir Michael Foster in his Text-book of physiology, 1877:

“Striking the tendon below the patella gives rise to a sudden extension of the leg, known as the knee-jerk.”

The term began to be used figuratively from the early 20th century onwards. O. O. McIntyre, in his New York Day-By-Day column in The Coshocton Tribune, October 1921, wrote this:

“Itinerant preacher stemming Broadway on a soap box. And gets only an occasional knee-jerk.”