The phrase "form a high opinion of oneself" can be described using several synonyms. One such synonym is the term "egotistical," which refers to a person who thinks highly of themselves and their abilities. Another synonym is "conceited," which indicates a self-centered attitude or an exaggerated sense of self-importance. Additionally, the term "arrogant" is a fitting synonym for someone who is self-absorbed and often dismissive of others. Other related synonyms include "self-important," "big-headed," and "pompous." In general, these words describe a person who sees themselves as superior to others, which can lead to negative interactions and perceptions from those around them.