A glass of hot toddy or gin-sling taken before going to bed at night. When a second glass is taken, it is called 'a string to tie it with.'
Come, now, Squire, before we turn in, let us tie the nightcap.--Sam Slick in England, ch. 3.
·noun A potion of spirit drank at bedtime. II. Nightcap ·noun A cap worn in bed to protect the head...
Webster's Dictionary of the English Language