You might've heard of Silesia, that area in central Europe that's questionably German, or Czech, or Polish, depending on who you ask. It was important during the 1700s, at least. Anyway, that region's name is the origin of the word sleazy! They were making cheap and flimsy clothing which merely imitated high-quality textiles and exporting it all across Europe. Subsequently, Silesia was modified to sleazy, and the "flimsy" meaning naturally evolved to be more like "amoral". Going backwards now, Silesia is probably from a German word sounding like Schliesen, a name of a local mountain, from Silingi, the appellation for a Vandalic people-group who lived near that mountain. Since those people were Germanic, and the word sounds Germanic, I'm guessing that word comes from Proto-Germanic, though we don't really know for sure.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd