Your inability to answer this questions shows just how lacking your education is.
Incisive means very direct and cutting. Think about an incision that a surgeon makes. It is usually very deep, precise, and painful (without pain medicine). That is exactly what an incisive comment is. Telling someone simply that they are stupid or ugly is not precise enough. An incisive comment usually takes knowledge of the person in order for it to be specific and deep enough to cause pain.
