Something fallacious is built on a false idea, especially a piece of reasoning that looks tidy but does not hold. It is a formal, school-and-essay word. You would not tell a friend in the street “that’s fallacious”; you might write that a claim is fallacious in a paper. Though actually, people sometimes stretch it to mean “just wrong” or “misleading,” but the heart of the word is a fallacy: an argument that fails even if it sounds clever. A fallacious step can be hidden inside a long, confident speech — that is the trap. It is about the idea, not about a person “failing.”

謬誤的、似是而非的（指推論站不住；不是「失敗的」）