Is it ensure, insure, or assure?
patellar reflex, neurological phenomenon discovered and named 1876; the figurative use appeared soon after the phrase was coined.
knee jerk n.
See patellar reflex.
: one more gesture to the knee-jerk hawks in the Congress/ Yet McDonald is no mere knee-jerk critic of the evangelicalsnoun
: We need less kneejerking on both sides in these arguments about the environment (1980s+)
[fr the patellar reflex, described as knee jerk by 1876]