The context is about the two roommates where, when one roommate returned home from work, the other roommate forced him to do the ‘onerous’ task of cleaning kitchen.
Here, onerous means something which causes burden. It maintain involving in a task which requires a lot of effort thus making it burdensome.
So when the roommate was already tired from his hectic day at work, then the other roommate forced him to engage in task of cleaning which he found onerous.
