The phrase "be of mind" refers to one's state of thinking, attitude or perspective towards something. The antonyms for this phrase are phrases that express opposite states of thinking, such as "be out of mind", which describes a lack of interest, neglect or abandonment, or "be of two minds", which refers to a state of indecision or uncertainty about something. Another antonym could be "be narrow-minded", which means having a limited or prejudiced perspective, while "be open-minded" would express a willingness to consider new ideas and perspectives. It's important to note that antonyms for "be of mind" will depend on the context in which the phrase is used.