The word "untaxing" refers to something that is easy, effortless, or not burdensome. Its antonyms, on the other hand, describe things that are difficult, arduous, or demanding. Some antonyms for "untaxing" include demanding, strenuous, challenging, formidable, taxing, laborious, tough, and exhausting. These words suggest that the task or activity requires significant effort, energy, or concentration. For instance, completing a complex project or solving a difficult problem can be considered as a demanding or challenging task that requires one's full attention and resources. Overall, the antonyms of "untaxing" demonstrate that things can be quite challenging and demanding in different contexts, and it is essential to put in significant effort to achieve success.