A posset is a type of sleeping draught, consisting of a warm beverage, usually wine or brandy, or milk mixed with the aforementioned spirits, and often laced with herbs or spices believed to have a sedative or even mildly narcotic effect, such as hops or nutmeg.
The manservant Ralph was bringing the Squire his nightly posset when he was felled by an energy beam fired by one of the Terileptils who had crash landed on the grounds of the Squire's estate outside Heathrow. (DW: The Visitation)