Not just cheap, but terrible liquor designed mostly to get you wasted and not much else. The term originated in the old west when many alcoholic beverages were designed purely to be easily afforded by anyone and their dog and to get the drinker hammered, often times, these cheap alcohols would make the drinker sick (due to the extremely low-quality of the ingredients). These days, the term just means something that's not much better in taste than rubbing alcohol and good for nothing more than getting you piss drunk when you're on a budget.
The single easiest way to detect rotgut is just by looking at the bottle. If it's in a plastic bottle, chances are good it tastes like piss mixed with gasoline. More often than not, anyone who claims to like rotgut is a poser teenager who thinks getting drunk makes them Billy Badass
Furthermore, a discerning drinker may consider anything below a certain standard of flavour as rotgut. If you can afford the "good" stuff, why the hell would you drink the bad stuff?
You want me to drink vodka from a plastic bottle? That stuff is rotgut, if they don't take the time to put it in a glass bottle, I won't take the time to drink it.