Posset \POS-it\ , noun;
1. A drink made of hot milk curdled with ale, wine, or the like, often sweetened or spiced
I don't know about you, but this sounds disgusting. The word dates to the mid-15th century and is of unknown origin. Posca has been suggested as a related word, but the Oxford English dictionary dismisses it on both semantic and formal grounds. Connections with Middle French possette and Irish posoid have also been suggested, but it seems that those words were probably borrowed from English, not the other way around.
After some Googling, it appears that posset is now a kind of dessert, sort of like a pudding. I also see that eggnog is considered a related item, so maybe calling it 'disgusting' was preemptive. Then again, there's no curdled milk in eggnog...