Maddening means so frustrating or annoying that it makes you feel like you might go crazy. When something is maddening, it bothers you intensely, often because it keeps happening or because you can't do anything to fix it.

A maddening sound is one you can't escape: the drip of a leaky faucet at night, or someone tapping their pencil during a quiet test. A puzzle can be maddening when you're stuck on the same part for the tenth time. Waiting can be maddening too, like when you're expecting important news and your parents keep saying “we'll tell you later.”

The word captures that specific feeling when frustration builds and builds. One drip isn't maddening, but the twentieth drip, when you're trying to sleep, is maddening. The word suggests something that tests your patience to its limit. A younger sibling who asks “why?” after every answer you give might be acting in a maddening way, even if they don't mean to annoy you.

Notice that maddening describes the thing causing the frustration, not the person feeling it. You don't say “I feel maddening.” You say “This is maddening!” when something drives you up the wall.