Source : Webster's Revised Unabridged Dictionary (1913)

Side line \Side line\
   1.
      (a) A line pert. or attached to the side of a thing.
      (b) Specif., a line for hobbling an animal by connecting
          the fore and the hind feet of the same side.

   2.
      (a) A line of goods sold in addition to one's principal
          articles of trade; a course of business pursued aside
          from one's regular occupation.
      (b) A secondary road; esp., a byroad at right angles to a
          main road. [Canada]