If one teacher gives you three hours of homework a night, that's rough. But if all of your teachers do it, that makes the task of completing your homework an onerous one, to say the least. If something is onerous, it is very difficult to deal with or do.
A near synonym is burdensome. In legal usage, onerous describes a contract or lease that has more obligations than advantages. Onerous derives from Middle English, from Old French onereus, from Latin onerōsus, from onus "burden." In English, an onus is a task or duty that is onerous, or very difficult.