Maddening describes something that makes you very annoyed — the kind of annoyed where you want to shout, but often you cannot fix it. A dripping tap, a phone that keeps buffering, a person who is almost ready but never quite ready: those are maddening. In ordinary talk it means “really, really annoying.” In slightly stronger or older use it can feel closer to “enough to make you lose your mind,” but most of the time it is not about real madness; it is about patience running out. It is an adjective (a maddening delay), not the action itself. It is common in writing and in careful speech; in very casual talk people may just say so annoying or driving me crazy.