What does 'knee-jerk reaction' mean?

What does knee-jerk reaction mean?

From NYT article:

Another senior Socialist, who declined to be identified, said the party could not afford knee-jerk reactions.

A knee-jerk reaction indicates a reflexive response.

There are a couple of particular concerns with such responses:

• They're spontaneous and generally involuntary
• Haste being necessarily so, these can often be detrimental to a situation, with no forethought as to the consequences

These are benefits in nature (if falling, notice that arms automatically attempt break the fall, particularly protecting the head); when applied to speech and whatnot, such as in office, it isn't a recommended practice to work solely from 'instincts' (for example, if a suggestion strikes a nerve with one member, emotions may speak for him, as opposed to taking an objective standpoint.)

It's an expression/saying meaning:

An immediate unthinking emotional reaction produced by an event or statement to which the reacting person is highly sensitive;

It's origin is thus:

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."

Reference:

http://www.phrases.org.uk/meanings/220600.html