Obligations are unavoidable, since they are responsibilities that have to be honored. Letting your grandmother pinch your cheek is unavoidable, and paying your rent is also unavoidable. As you get older, it's unavoidable that your knees will get stiffer — there is no way to avoid having this happen. Unavoidable comes from the verb avoid, which is rooted in the Anglo-French avoider, "to clear out or withdraw oneself."