Grandpa's old cough syrup usually refers to a hot toddy. A drink made with one shot of whiskey, a half cup of boiling water, a spoonful of honey, and a splash of lemon juice. Variations include adding cloves and/or cinnamon sticks then letting the drink brew for approximately 5 minutes before serving.
The nickname of Grandpa's old cough syrup comes from the hot toddy being used as a cold/flu remedy.
You guys been doing a little bit of boozing? Sucking back on grandpa's old cough syrup?