A "kneejerk reaction" refers to an immediate and automatic response to a situation without thinking or evaluating it beforehand. It is often impulsive and instinctive, like a reflex when you hit your knee with a hammer. Full definition
The nuances of coalition mean that thankfully such kneejerk reactions are no longer possible. (politics.co.uk)
Michael Kozlowski, you sir are doing a great disservice by helping to propagate ridiculous misinformation and an even more ridiculous kneejerk reaction. (goodereader.com)
That way, you'll be less likely to make an uninformed kneejerk reaction to news that turns out to be exactly the wrong move. (fool.com)