Answer :
Here is a list of words for 'without thinking' : blindly, thoughtlessly, instinctively, carelessly, recklessly, irrationally and there are some other words for that too.
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'
Some people say that it is better to 'think before you speak'