You didn't think about it beforehand. The idea expressed is that someone did or said something, but before acting or speaking, he didn't think about what he was going to do or say. That is, he acted or spoke without thinking.

It is often said about a gift someone got for a friend. If they just didn't 5hink about what the friend might like, but instead just grabbed some random gift we would say that he didn't put much thought into the gift. So he didn't really care much about it.