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]

The Collaborative International Dictionary of English v.0.48:

Sideline \Sideline\, v. t.
   To render unable to participate in an activity, such as a
   sporting event; as, the pitcher was sidelined by a groin
   injury.
   [PJC]

WordNet (r) 3.0 (2006):

sideline
    n 1: a line that marks the side boundary of a playing field
         [syn: sideline, out of bounds]
    2: an auxiliary line of merchandise
    3: an auxiliary activity [syn: avocation, by-line, hobby,
       pursuit, sideline, spare-time activity]
    v 1: remove from the center of activity or attention; place into
         an inferior position; "The outspoken cabinet member was
         sidelined by the President"