What is a nightcap?

Answer 1

A "nightcap" is an alcoholic drink before bed. People used to put on actual caps before going to bed, because they didn't have heating inside (except for a fireplace), so they got cold. Alcohol seems to warm you up (it really doesn't, but it seems to, which is how this idiom got started) -- people started referring to an alcoholic drink at bedtime as a nightcap because it got you feeling warm just like your actual nightcap did.

Answer 2

A nightcap is either a piece of headwear worn while sleeping, or a beverage, usually alcoholic, drunk before going to bed.

In the world of Baseball, it can also refer to the final match of a sporting contest, such as the second match of a baseball doubleheader.

Answer 3

A nightcap is an beverage (can be alcoholic in most cases) that you drink before you go to bed.