Knee jerk reflex

A contemporary coinage by the press, I suppose, beats my brains in establishing any possibility of a coherence in between the constituent words and the meaning. What the duece did they imagine would be obvious in tying the word knee-jerk to the meaning "automatic" or "predictable".

PS: My memory fails me, what are each of the words (the two nouns or a noun and an adjective as in ) in a hyphenated phrase individually known as?

I'm sure that you're familiar with the image of a doctor tapping a patient just below the knee and the leg kicking out. It demonstrates the knee jerk or patellar reflex.