The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
A sudden involuntary forward movement of the lower leg that can be produced by a firm tap to the tendon located just below the kneecap.
The term is loosely applied to any response or belief that is automatic rather than thoughtful. In this sense, it is usually a term of reproach: “Smith is a knee-jerk liberal.”
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.