The Collaborative International Dictionary of English v.0.48:

Sundown \Sun"down`\, n.
   1. The setting of the sun; sunset. "When sundown skirts the
      moor." --Tennyson.
      [1913 Webster]

   2. A kind of broad-brimmed sun hat worn by women.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sundown
    n 1: the time in the evening at which the sun begins to fall
         below the horizon [syn: sunset, sundown] [ant:
         aurora, break of day, break of the day, cockcrow,
         dawn, dawning, daybreak, dayspring, first light,
         morning, sunrise, sunup]