: a drink that you have just before you go to bed at night and that usually has alcohol in it.

What does nightcap slang mean?

Informal. an alcoholic drink taken at bedtime or at the end of a festive evening.

Why do people drink night caps?

A nightcap was originally alcoholic, since it makes the drinker feel warm and helps them sleep, just like the garment of the same name. However, warm milk is often recommended as a nightcap for inducing sleep. It contains both tryptophan and calcium, which both help induce sleep.