It smells absolutely rotten and offensive. Birds are falling from the sky because of the fumes. You might throw up from one whiff. Whatever it is, it reeks.
Shakespeare famously wrote about the "breath that from my mistress reeks," but that's not a nice way to talk about a girlfriend. The word usually describes smells, but can also be used to suggest something fishy, as in "the suspicious transactions reeked of mob involvement." But noses everywhere agree: Whether it's coming from your sneakers or from the aftermath of a shady deal, if something reeks — it stinks to high heaven.