Maddeningly is the adverb: it tells you that something is done, or something is true, in a way that makes people very annoyed. You put it with an adjective or another adverb more often than on its own: maddeningly slow, maddeningly close, maddeningly calm (when you wanted them to hurry). The picture is usually this: the thing is almost all right, or it is happening, but the way it happens wears you down. “Maddeningly close” means you nearly got there, and that near-miss is what stings. It does not mean “in a mad way” (wildly, crazily). It means “in a very annoying way.” It is a little formal or writerly; in speech people often say annoyingly or just so slowly it drove me mad.

氣人地、令人火大地（多與形容詞連用，如 maddeningly slow）