Top out refers to the highest level that something can achieve, such as a mountain climber reaching the summit of a peak. The opposite or antonym of top out could be bottom out, which represents the lowest point or level that something can reach. Other antonyms that could apply to top out include underachieve or fall short, which refer to not meeting expectations or failing to reach a desired level of success. Another antonym could be plateau, which represents a level of stability or lack of progress, as opposed to the growth and achievement represented by reaching the top.