Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. A potion of spirit drank at bedtime. [Cant] --Wright.

Source : WordNet®

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