A warm cloth cap worn while sleeping, often with pajamas, being common attire in northern Europe before effective home heating became widespread.
Winston wore a nightcap to stave off the cold.
A beverage drunk before bed that is usually alcoholic.
I'll make myself a nightcap of whisky and lemon before heading to bed.
(by extension) Something the person reads or listens to before bed.
The final match of a sporting contest, especially the second game of a baseball doubleheader.
A cap drawn over the face of the condemned person before they are hanged.