Mild mannered is a term that is generally used to describe someone who is calm and gentle in nature. It is often used as a compliment to describe a person who is easy-going and polite. However, if one wants to describe the opposite of mild-mannered, a few antonyms come to mind. Some of the antonyms for mild-mannered are aggressive, hostile, hot-tempered, irritable, and violent. These adjectives describe someone who has a temper and is quick to react angrily. While mild-mannered may be seen as a positive trait, these antonyms are often used to describe someone who is negative and unpleasant to be around.