A nightcap is a cap worn in bed to keep one’s head warm. The word is first recorded in the description and valuation, made in 1378, of the articles that were in the shop of Thomas Trewe, haberdasher of London: one dozen of white caps, called “nightcappes”, was worth 2s. 3d..

The figurative use of nightcap in the sense of a hot or alcoholic drink taken before bedtime is often a jocular euphemism for something alcoholic, which also warms one. But it may have originally arisen because a late-night drink and the donning of a nightcap were both associated with preparing to go to bed.

This figurative use seems to date from the early 19^th century. It is first recorded in Three Rooms on a Floor or Clerical Comfort at an Inn (7^th March 1814), a satirical print by Charles Williams (active 1796-1830):

image: The British Museum

The historian Mary Dorothy George (1878-1971) thus described this engraving in Catalogue of political and personal satires preserved in the Department of Prints and Drawings in the British Museum (volume 9 – 1949):

Three designs side by side, representing sections of three adjacent rooms. In the middle a parson with a carbuncled nose lies full-face, with closed eyes and clasped hands. His clerical hat, wig, and coat hang from a peg. On the floor are his clumsy shoes and a portmanteau labelled ‘Dr Drowsey Brazen Nose Oxford’. He says: “Bless me!—how the Gentlemen in the next rooms do swear, a pretty place this for a man of my cloth!!”— On the left a man sits up in bed, a bowl of ‘Grog’ beside him on his sea-chest, which is labelled: ‘Lieutenant Bowling of the Tremendous Frigate’. With raised arm and clenched fist he declaims: “O that glorious engagement.—D—nation, if ever I can sleep for thinking of it”. His sabre and buckled shoes are beside the low bed. On the right a burly whiskered fellow sits up in bed yelling: “Hollo Chambermaid—you have forgot my nightcap—By the holy St Patrick if you dont [sic] bring it directly may perdition sieze [sic] me if I dont fire the House”. Beside him are a chest inscribed ‘Capt O’Callagan Irish Brigade’, plumed cocked hat, sword, boots, and pistol.

In the 1818 edition of Apicius Redivivus; Or, The Cook’s Oracle, the English optician, inventor of telescopes, amateur musician and cook William Kitchener (1775-1827) wrote (nightcap is in quotation marks, which seems to indicate that the figurative use was recent):

Tewahdiddle.

A pint of table beer, (or Ale, if you make it for a “Night-Cap,”) a tablespoonful of brandy, and a teaspoonful of brown sugar, or clarified syrup; a little grated nutmeg or ginger may be added, and a roll of lemon-peel.
Obs.—Before our readers make any remarks on this composition, we beg of them to taste it; if the materials are good, and their palate does not differ very much from that of its inventor, they will find it one of the most delicious beverages they ever put to their lips.

In the first (1817) edition of this book, this recipe thus begins (the word nightcap does not appear):

A pint of table beer, a tablespoonful of brandy, two teaspoonful of brown sugar, or clarified syrup…

There are three instances of night-cap ale in The Suffolk Chronicle; Or, Weekly General Advertiser, and County Express in 1814. The first one is in the 18^th-June edition:

Bungay Dinner.—On Monday last, Matthias Kerrison, Esq. of Bungay, regaled at his house a number of poor children with a plentiful dinner of beef, mutton, night-cap ale, &c. for which they made the most becoming acknowledgments.

The second one is in the 2^nd-July issue:

Ditchingham Peace Festival.—On Friday week the late joyful occurrences were celebrated at Ditchingham, near Bungay, with the utmost gratitude, hilarity, and decorum. Tables being arranged and cloths laid thereon, in the Hall Park, nearly 500 cottagers sat down, about one o’clock, and partook of a plentiful dinner of plum-puddings and roast beef, with 9 barrels of fine night-cap ale. Nearly the whole afternoon was devoted to the exhibition of rural and ludicrous sports—pig-hunting, jumping and running in sacks, races for money, distortions of countenance, &c. continued till nearly 9 o’clock, when, by moonlight, the lads and lasses of the village struck up many country dances, to the high gratification of those who had so liberally contributed towards the enjoyment of the day. The principal donors’ health having been given, with three times three, they retired peaceably to their respective homes. The next day a distribution of fragments took place.

The third mention is in the 13^th-August edition; an article titled Peace Festivals thus begins:

Mettingham.—Though nearly the last in succession, the Mettingham Festival may truly be classed amongst the first for comfort, jocundity, and decorum.—Yesterday week, by the liberal contributions of John Bond, Esq. and the principal inhabitants, all the poor cottagers were plenteously feasted with plum-puddings and roast beef, with an abundant supply of “Night-cap Ale.”