Incisiveness is the quality of being incisive — that sharp, clear, no-waste way of seeing or saying something. You can praise the incisiveness of an argument, a question, or a person’s mind. It is a noun, so it names the quality, not a single remark. Formal English; in a casual chat someone might just say “she’s really sharp” or “she gets straight to the point.” The word still carries a faint picture of a blade, but in modern use it is almost always about intelligence and clarity, not about cutting skin or wood.