Something that's tiresome is extremely dull or annoying, like the tiresome lecture your whole class gets every time some forgets to turn in their homework.
Your little sister might love her enormous picture book about trucks, while you find it tiresome (especially when you're reading it to her for the tenth time). It's entirely possible that she finds some of your favorite things—like French movies, baking pies, or jazz—equally tiresome. This synonym for tedious first appeared around 1500, from the verb tire, or "become weary."