The Collaborative International Dictionary of English v.0.48:

Backset \Back"set`\ (b[a^]k"s[e^]t`), n. [Back, adv. + set.]
   1. A check; a relapse; a discouragement; a setback.
      [1913 Webster]

   2. Whatever is thrown back in its course, as water.
      [1913 Webster]

            Slackwater, or the backset caused by the overflow.
                                                  --Harper's
                                                  Mag.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Backset \Back"set`\, v. t.
   To plow again, in the fall; -- said of prairie land broken up
   in the spring. [Western U.S.]
   [1913 Webster]