Combine all ingredients except the rhubarb in a medium saucepan and bring to a boil, stirring to dissolve the sugar and salt and cook off the alcohol.
Cool completely (if you're in a hurry, put the pan in an ice bath). Pack the sliced rhubarb into a quart-sized glass jar. When the liquid is fully chilled, pour over the rhubarb.
Refrigerate for at least 2 hours. These will keep for 3 weeks or more.