Question

What's the word for; a response without thought

Answer 1

Here is a list of words for 'without thinking' : blindly, thoughtlessly, instinctively, carelessly, recklessly, irrationally and there are some other words for that too.

Answer 2

I suppose you could just say it was an immediate reaction which is careless/thoughtless.
Some people say that it is better to 'think before you speak'