If you are being deductive, you use the knowledge and information you already have, and from that you work out what something means, or form an opinion. You are not guessing in the dark, and you are not rushing out to gather a whole new pile of examples first; you start from what is in your hand. In school this word often sits beside inductive: inductive thinking walks from many small cases up toward a general idea, and deductive thinking walks the other way, from a rule or a set of facts down to a conclusion about one case. Though actually, outside the logic classroom people also call it deductive when someone is simply good at putting clues together, the way a detective does — that everyday use is looser than the tidy school contrast. It is a serious, somewhat formal adjective. You will meet it in phrases like deductive reasoning or deductive skills; friends almost never say “that was deductive of you” in ordinary chat.