NOUN (3)

1. an alcoholic drink taken at bedtime; often alcoholic;

2. a cloth cap worn in bed;

3. the final game of a double header;

The Collaborative International Dictionary of English v.0.48:

Nightcap \Night"cap`\, n.
   1. A cap worn in bed to protect the head, or in undress.
      [1913 Webster]

   2. An alcoholic beverage drunk at bedtime. [Cant] --Wright.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nightcap
    n 1: an alcoholic drink taken at bedtime; often alcoholic
    2: a cloth cap worn in bed
    3: the final game of a double header

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "nightcap":
   Mickey, Mickey Finn, anesthetic, aperitif, chaser, cocktail,
   doch-an-dorrach, drink, eye-opener, highball, hypnotic,
   knockout drops, lullaby, mandrake, mixed drink, morphia, morphine,
   opiate, opium, parting cup, poppy, pousse-cafe, punch, sedative,
   sleep-bringer, sleep-inducer, sleep-producer, somnifacient,
   soporific, stirrup cup, sundowner, wee doch-an-dorrach