Not thinking about or caring about how your actions affect other people's feelings or needs.
From Latin 'in-' (not) + 'consideratus' (thought about carefully). The word evolved in English to describe someone lacking thoughtfulness toward others, combining the prefix negation with the concept of careful consideration.
This word became especially common in the 1800s as modern society required more awareness of strangers' feelings—back when people lived in smaller groups, you had to face the consequences of rudeness directly, so the concept wasn't as necessary!
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