Noun

1. A wandering traveller/worker (such as a swagman) who arrived at a farm too late in the day to do any work, but still partook of their hospitality.
   • 1846: C. P. Hodgson, Reminiscences of Australia
   • 1868: Thomas E. Spencer, The Song of the Sundowner http://oldpoetry.com/poetry/28271
2. (nautical) A harsh disciplinarian sea captain typified by requiring all hands on board by sundown. http://www.tpub.com/content/administration/12966/css/12966_381.htm
   • 1985: Ronald H. Spector, Eagle Against the Sun

     Arrogant, aloof, and suspicious, a "sundowner," or strict disciplinarian, King inspired respect in many but affection in few.

3. A patient, usually demented, who tends to become agitated in the evening.
   • 1977: Jules Hymen Masserman, Current Psychiatric Therapies, page 179.

     These patients may improve by day only to relapse at night (nocturnal delirium or sundowner's syndrome).

   • 1989: William H. Reid, The Treatment of Psychiatric Disorders: Revised for the DSM III R., Page 71. http://books.google.com/books?vid=ISBN0876305362

     They generally occur in the evening or at night in the form of "sundowner" syndrome, as a result of diminished sensory input and social isolation and/or exposure to an unfamiliar environment (e.g., the hospital).

   • 2007: Dennis Fiely, Dark Ages: For the elderly fighting mental or physical problems, life takes a frightening turn when nighttime comes. The Columbus Dispatch, Wednesday, February 07, 2007. http://www.dispatch.com/dispatch/contentbe/dispatch/2007/02/07/20070207-B1-00.html

     "Sundowner's syndrome" refers to changes in mood and behavior that begin near dusk.

4. A cocktail consumed at sunset, or to signify the end of the day. A cocktail party in the early evening.
   • 1918: Robert Valentine Dolbey, Sketches of the East Africa Campaign, page 117.

     The cocktail, the universal "sherry and bitters" and sundowner will have to be retained.

   • 2005: Franz Wisner, Honeymoon With My Brother: A Memoir, page 243.

     Per custom, we capped our drives with a sundowner cocktail party at a scenic vantage point.

   • 2005: Edward M. Bruner, Culture on Tour: Ethnographies of Travel, page 83.

     The Sundowner is basically a cocktail party with a buffet on a riverbank in the bush.