To kneejerk is to snap out a reply or an action before you have really looked at the situation. It comes from the doctor's test: a tap just below the knee, and the leg kicks by itself — you cannot stop it. So a kneejerk 'no', or a kneejerk protest, is automatic, not thought through. You will meet this far more often as an adjective: a knee-jerk reaction, knee-jerk opposition. The spelling usually takes a hyphen (knee-jerk). It is slightly critical: the speaker thinks you should have paused. Common in news and argument. Though actually it is not a medical word when people use it this way; that old reflex is only the picture behind it.