The antonyms for the phrase "speaking highly of" are "criticizing," "disparaging," "condemning," "belittling," "ridiculing," and "blaming." These words suggest a negative opinion of something or someone, and may be used to express disapproval or displeasure. Example sentences include "Instead of speaking highly of his work, he is always criticizing it," or "She was constantly disparaging her colleagues, which made for a toxic work environment." These antonyms emphasize the importance of being respectful and honest in our communication, and avoiding unnecessary negativity towards others.