Something is toilsome if it's really difficult, requiring exhausting or boring effort. Shoveling a foot of heavy snow out of your neighbor's long driveway is toilsome.
The adjective toilsome is archaic — it's hardly ever used anymore. It's a shame, because it's a good way to describe monotonous or strenuous work, like your toilsome math homework or your sister's toilsome job on a construction site. At the heart of toilsome is the verb toil, or "work," from the Old French toeillier, "drag about or make dirty," which is probably rooted in the Latin word tudiculare, "crush with a small hammer."