When someone speaks, argues, or assumes fallaciously, they are doing it in a way that rests on a false idea. Same formal feel as fallacious, only now it tells you how the action is done: “He fallaciously treated one survey as if it spoke for the whole country.” Learners meet the adjective far more often than this adverb. Though actually, in speech people usually just say wrongly or on a false idea; fallaciously belongs to careful writing. It still points at the reasoning, not at “failing.”

謬誤地、似是而非地