The Collaborative International Dictionary of English v.0.48:

Sideline \Sideline\, Side line \Side line\n.
   1.
      (a) A line pertaining 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.
          [Webster 1913 Suppl.]

   2. 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.
      [Webster 1913 Suppl.]

   3. A secondary road; esp., a byroad at right angles to a main
      road. [Canada]
      [Webster 1913 Suppl.]

   4. (Sports) Either of two marked lines which delimit the side
      boundaries of a playing field, as in football or soccer.
      [PJC]

   5. (Sports) [plural] The areas in a sports arena just beyond
      the sidelines[4], where coaches and alternative players
      sit or stand.
      [PJC]

   on the sidelines not actively involved in an activity, such
      as a sporting event, but available for use when needed; in
      reserve; as, the coach kept him on the sidelines for a
      week.
      [PJC]