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]

U.S. Gazetteer Places (2000):

Sundown, MO -- U.S. village in Missouri
   Population (2000):    38
   Housing Units (2000): 25
   Land area (2000):     1.072340 sq. miles (2.777347 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    1.072340 sq. miles (2.777347 sq. km)
   FIPS code:            71628
   Located within:       Missouri (MO), FIPS 29
   Location:             36.568280 N, 92.637779 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Sundown, MO
    Sundown


U.S. Gazetteer Places (2000):

Sundown, TX -- U.S. city in Texas
   Population (2000):    1505
   Housing Units (2000): 575
   Land area (2000):     1.512635 sq. miles (3.917706 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    1.512635 sq. miles (3.917706 sq. km)
   FIPS code:            71060
   Located within:       Texas (TX), FIPS 48
   Location:             33.457018 N, 102.488283 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Sundown, TX
    Sundown