The phrase "acquit oneself" means to perform a task or responsibility successfully. Some antonyms that could be used in place of "acquit oneself" include "fail," "fall short," "underperform," "blunder," "botch," or "mismanage." These words all indicate an inability to successfully complete a task, whereas "acquit oneself" implies a successful completion or execution. Additionally, other antonyms such as "neglect," "avoid," or "shirk" imply a lack of effort or responsibility taken altogether, rather than an inability to perform well. Overall, there are many different words that carry a different connotation than "acquit oneself" and could serve as suitable antonyms.