When something happens signally, it happens in a very clear, strong way — everyone can see it. It is not a little bit; it stands out. Writers use it with words like fail, succeed, or lack: a plan that “failed signally” failed so plainly that nobody can pretend otherwise. It is a careful, rather old written word, not a spoken one. Younger people almost always say clearly, obviously, or badly instead. Though actually, the signal inside it is not a traffic light. It is the older, bookish sense of signal meaning “striking, easy to notice.” Signally is that idea turned into an adverb.