If something is dingy, it's dirty. If you spend your days as a chimney sweeper, you probably look pretty dingy.
The adjective dingy is often, but not always, used to describe one's clothing or living space. The adjective dingy comes from uncertain origins, but experts suspect it may be a backformation from the word dung, which is animal excrement. So you can imagine how dirty, dismal, grungy, and grimy something described as dingy is.