Something crooked is not straight, like a trail through the woods, or your cousin's unfortunate set of teeth.
Crook is a Middle English verb meaning "bend," which comes from the Norse for hook. So crooked means "bent out of shape or curving around sharply." A picture hanging diagonally on the wall is crooked, and so is a hunchback's spine. We also use crooked metaphorically to talk about being bent out of moral shape, so to speak. A crooked politician is dishonest and takes bribes, and crooked businessmen steal from unsuspecting clients.