What term describes someone who is hasty and unthinking, lacking in tact or sensitivity?

The term that best describes someone who is hasty and unthinking, lacking in tact or sensitivity, is "brash." This word conveys a sense of someone who acts with reckless confidence, often without considering the consequences of their actions or the feelings of others. A brash person tends to speak or behave in a way that can come across as rude and impetuous, demonstrating a lack of discretion and care in their interactions.

In contrast, the other terms provided refer to different characteristics. "Cantankerous" describes someone who is irritable and difficult, generally more about mood rather than acting without thought. "Brittle" often refers to something that is fragile or easily broken, and does not apply to personality traits. "Benign" suggests something that is harmless or gentle, which also does not fit the description of being hasty or lacking sensitivity. Therefore, "brash" is the most appropriate choice for the given definition.