A small drink of alcohol consumed just before bedtime, or a hat worn at night in bed.
From 'night' plus 'cap.' Originally referred only to a sleeping hat worn to keep the head warm, then evolved to mean a final drink before sleep—probably because people wore nightcaps while having bedtime drinks.
Victorians believed nightcaps (the hats) were essential for proper hair care and brain health—some people slept in them well into the 1950s, thinking they'd wake up refreshed and handsome!
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