The definition of un-taxing is something that is easy, effortless, and does not require much thought or energy. Therefore, some antonyms for un-taxing could be challenging, difficult, strenuous, laborious, complicated, intricate, arduous, burdensome, complex, demanding, hard, onerous, stressful, and taxing. These words are used to describe tasks that require a considerable amount of effort, time, and energy to complete. Whether it is a physical or mental task, these antonyms of un-taxing represent activities that challenge an individual's capabilities and abilities. Ultimately, the antonyms of un-taxing indicate a state of difficulty and complexity that requires diligence and effort.