nightcap

A nightcap was originally a cloth cap worn to bed. In unheated medieval and early modern bedrooms, the head lost heat fastest. A linen or wool cap kept the sleeper warm. The word is a straightforward compound: night + cap. Nightcaps appear in English texts from the fourteenth century onward, and they were standard bedwear for men and women across social classes for roughly five hundred years.

By the seventeenth century, the word acquired its second meaning. A drink taken before bed — usually warmed ale, wine, or spirits — was called a nightcap because it served the same function as the cloth one: it warmed you before sleep. The metaphor was direct and physical. Samuel Pepys recorded drinking nightcaps in his diary in the 1660s. The transition from headwear to beverage was seamless because the purpose was identical.

The cloth nightcap disappeared in the nineteenth century as central heating spread through European and American homes. The drink survived. By the twentieth century, a nightcap was exclusively a drink — brandy, port, whisky, or any spirit taken as a last drink of the evening. The hat meaning was forgotten so thoroughly that most English speakers today do not know the drink was named after headwear.

In contemporary English, 'come up for a nightcap' carries a social subtext that has nothing to do with either hats or warm drinks. The word has gained a third layer of meaning — an invitation that means itself and also means something else. A fourteenth-century sleeping cap became a seventeenth-century warm drink became a twenty-first-century euphemism.