Rotgut is strong, cheap, low-quality alcohol — the harsh kind people joke will wreck you. The word is a picture: it would rot your gut, your stomach. It is informal, a bit old-fashioned, and rough-funny. You might hear it about bad whiskey, cheap spirits, or homemade drink. Nobody puts rotgut on a menu; it is a complaint or a joke, not a scientific name. Though actually, it does not name one exact drink. It names a quality, or a lack of quality. Wine people would not usually call a poor wine rotgut; the word leans toward strong spirits. It can sound American. Use it with a grin, not as a polite order at a nice bar.