If a person or an act is despicable, they are so low, so cruel, or so wrong that decent people look down on them — cheating someone weaker, kicking a person who is already down, hurting for sport. It is a strong moral word, a bit formal, but people still reach for it when they are truly angry. In loose talk someone might just say “that's awful” or “what a jerk”; despicable makes the judgment harder. It almost always describes a person or an act. You will sometimes see it used as a noun for a person (a despicable), but that is rare and sounds like writing, not speech. Though actually, everyday English often uses it for things that are mean rather than truly evil — a cheap trick, a nasty comment — so it has stretched a little from “monster” toward “really not okay.” It is not a light joke word, even when people stretch it. You despise what is despicable: the two words grew from the same “look down on” idea.