(sarcasm) Placing a napkin on top of a drink at a bar to indicate that you
do not want a ruffie (Rohypnol) put in your drink while it's unattended.
Meant to be said in a sarcastic manner. Clearly this would not stop someone from putting something in your drink. Useful as a way to maintain awareness that you should always watch your drink without sounding like you are someone's mother.
Most often used to indicate to
the bartender that the patron will be returning (so they don't throw your drink away) and/or that the seat is occupied. Typically done when a patron is using
the bathroom or outside smoking a cigarette.