Uncompassionate means indifferent or uncaring about the way other people feel. An uncompassionate person isn't emotionally affected by the sight of someone who's crying.
An uncompassionate teacher won't accept any excuse for your lateness, even if you're upset about your sick dog. Uncompassionate laws punish people without regard for the seriousness of the crime or the reason behind it. This adjective is formed by adding un-, "not," to compassionate, "feeling sympathy or concern for others."