Unthinking describes words or actions that come out with no thought behind them — especially no thought about who might get hurt. An unthinking remark is one the speaker never meant badly; they simply did not stop and consider. That is the heart of the word: it blames carelessness, not cruelty. Calling someone unthinking is a real criticism, but a much gentler one than calling them cruel or selfish.

It is used two ways. Describing people or their words: an unthinking comment, unthinking rudeness, an unthinking child. And describing behaviour done automatically, like a machine: unthinking obedience, unthinking loyalty, the unthinking acceptance of rules. This second use is stronger and often political — it means people follow along without ever asking whether it is right.

The word goes before a noun almost every time. You would rarely say 'he was unthinking'; you would say 'it was an unthinking thing to say'. It belongs to writing and careful speech more than to chat. In everyday talk people say 'thoughtless' — which means the same thing and is much more common.