To vex means to annoy or frustrate someone in a way that really gets under their skin. When something vexes you, it's more than a minor irritation: it bothers you repeatedly or makes you feel genuinely frustrated.

A teacher might be vexed by students who keep talking during quiet reading time, day after day. You might feel vexed by a shoelace that keeps coming untied no matter how carefully you tie it, or by a math problem that seems simple but somehow keeps giving you the wrong answer. The word suggests persistent annoyance rather than sudden anger.

Something that causes this kind of frustration is called vexing. A vexing question is one that keeps puzzling you. A vexing situation is one that continues to create problems no matter what you try.

The word has an old-fashioned feel to it. Instead of saying “That really bugs me,” someone might say “It vexes me greatly.” The feeling itself is timeless, though: that particular kind of frustration when something keeps bothering you and won't let up.