The phrase "be mad about" typically means to be very fond or passionate about something. However, there are several antonyms that convey the opposite meaning. To be indifferent, uninterested, or apathetic are some possible antonyms for "be mad about." If someone is indifferent to something, they lack any strong emotions or feelings towards it. Uninterested means a person has no interest in something, and finally, apathetic means a disregard or lack of passion towards something. So, depending on the situation, these antonyms can be used to communicate the opposite of "be mad about.