Dredging is digging up mud, sand, or waste from the bottom of a river, lake, or harbor, usually with a big machine on a boat — done to make water deeper for boats to pass through, or to clean up pollution. There's also a figurative use, "dredging up," which means bringing back an old memory, story, or piece of information that people had mostly forgotten — often something unpleasant, like "dredging up old arguments."