Source : Webster's Revised Unabridged Dictionary (1913)

Blackstrap \Black"strap`\ (-str[a^]p), n.
   1. A mixture of spirituous liquor (usually rum) and molasses.

            No blackstrap to-night; switchel, or ginger pop.
                                                  --Judd.

   2. Bad port wine; any common wine of the Mediterranean; -- so
      called by sailors.