you could be referring to a schnapps or liqueur that you drink after a meal to aid digestion, or you could be asking if a friend wants a final drink of the evening before you both head home. In the latter sense of a farewell drink, ein Absacker is comparable to the English "nightcap", which denotes the last drink before bed. In this sense, an Absacker is a drink that relaxes you and brings your energy levels down after the excitement of the night, preparing you for your departure and a relaxing night's sleep (provided you don't overdo them,, that is!