When you're sad, you feel unhappy. If you've ever experienced the death of a pet you loved deeply, you know exactly what it means to feel sad.
You might use the adjective sad informally to describe something that's pathetic or that you feel scornful or disdainful about. For example, you might comment on your friend's elaborate homemade hat by saying, "That hat is just sad," though it wouldn't be very nice of you to say it. The term "sad sack" became popular during World War II and has been used ever since to refer to an incompetent, slightly pitiful person.