Informal.an alcoholic drink taken at bedtime or at the end of a festive evening.
2.
a cap for the head, intended primarily to be worn in bed.
3.
Sports Informal.the last event of the program for the day, especially the second game of a doubleheader in baseball or the last race of the day in horse racing.
late 14c., "covering for the head, worn in bed," from night + cap. In the alcoholic sense, it is attested from 1818. Amer.Eng. sense of "final event in a sporting contest" (especially the second game of a baseball double-header) is from 1939.