A swain is a lad, youth, or man. By extension, a yuoung man in charge of boats on a ship is a boat's swain, which became "bos'n" in modern sailor talk. You occasionally also see references to herd swains, but for some reason not as often.

1. A servant. [Obs.]

         Him behoves serve himself that has no swain.
                                              --Chaucer.

2. A young man dwelling in the country; a rustic; esp., a
   cuntry gallant or lover; -- chiefly in poetry.

         It were a happy life To be no better than a homely
         swain.                               --Shak.

         Blest swains! whose nymphs in every grace excel.
                                              --Pope.


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