The word "study at onerous" implies a level of difficulty and challenge when it comes to learning or education. There are several synonyms that could be used to convey this same meaning, such as "burdensome," "taxing," "arduous," "challenging," or "demanding." Other words that could be used to describe the same situation might include "diligent," "grueling," "strenuous," "onerous," or "draining." Regardless of which word is used to describe this situation, it is clear that the task at hand requires a significant amount of effort, energy, and hard work in order to achieve the desired results.