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"

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "sideline":
   avocation, bank, board, border, bordure, brim, brink, brow,
   by-line, catalog goods, coast, commodities, consumer goods,
   consumer items, delay, edge, effects, featheredge, flange, frame,
   fringe, goods, goods for sale, hem, hobby, hobbyhorse, inventory,
   job lot, labellum, labium, labrum, lay aside, lay away, lay by,
   ledge, limb, limbus, line, line of goods, lip, list,
   mail-order goods, marge, margin, merchandise, pastime, pigeonhole,
   postpone, push aside, put aside, put away, put in mothballs,
   ragged edge, rim, selvage, set aside, set by, shelve, shore, side,
   side interest, skirt, spare-time activity, staples, stock,
   stock-in-trade, store, stow, table, table the motion, vendibles,
   verge, wares