The Collaborative International Dictionary of English v.0.48:

Side \Side\, v. i. [imp. & p. p. Sided; p. pr. & vb. n.
   Siding.]
   1. To lean on one side. [Obs.] --Bacon.
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   2. To embrace the opinions of one party, or engage in its
      interest, in opposition to another party; to take sides;
      as, to side with the ministerial party.
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            All side in parties, and begin the attack. --Pope.
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The Collaborative International Dictionary of English v.0.48:

Siding \Sid"ing\, n.
   1. Attaching one's self to a party.
      [1913 Webster]

   2. A side track, as a railroad; a turnout.
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   3. (Carp.) The covering of the outside wall of a frame house,
      whether made of weatherboards, vertical boarding with
      cleats, shingles, or the like.
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   4. (Shipbuilding) The thickness of a rib or timber, measured,
      at right angles with its side, across the curved edge; as,
      a timber having a siding of ten inches.
      [1913 Webster]

WordNet (r) 3.0 (2006):

siding
    n 1: a short stretch of railroad track used to store rolling
         stock or enable trains on the same line to pass [syn:
         siding, railroad siding, turnout, sidetrack]
    2: material applied to the outside of a building to make it
       weatherproof

Moby Thesaurus II by Grady Ward, 1.0:

139 Moby Thesaurus words for "siding":
   L, adobe, ashlar, bank, beam, billet, board, boarding, border,
   branch, brick, bricks and mortar, broadside, cable railway, cement,
   cheek, chop, clapboard, clinker, coast, cog railway, concrete,
   cord, cordwood, covering materials, deal, driftwood, el,
   electric railway, elevated, elevated railway, embankment, feeder,
   feeder line, ferroconcrete, firebrick, firewood, flag, flagstone,
   flank, flooring, gravity-operated railway, hand, handedness,
   hardwood, haunch, hip, horse railway, jowl, junction, laterality,
   lath, lath and plaster, lathing, lathwork, light railroad, line,
   log, lumber, main line, many-sidedness, masonry, metro, monorail,
   mortar, multilaterality, panelboard, paneling, panelwork, pavement,
   paving, paving material, plank, planking, plasters, plyboard,
   plywood, pole, post, prestressed concrete, profile, puncheon,
   quarter, rack railway, rack-and-pinion railway, rail, rail line,
   railroad, railway, roadbed, roadway, roofage, roofing, shake,
   sheathing, sheathing board, sheeting, shingle, shore, side,
   sideboard, sidetrack, slab, slat, softwood, splat, stave, stick,
   stick of wood, stone, stovewood, street railway, streetcar line,
   subway, switchback, temple, terminal, terminus, three-by-four,
   tile, tiling, timber, timbering, timberwork, track, tram, tramline,
   trestle, trolley line, trunk, trunk line, tube, turnout,
   two-by-four, underground, unilaterality, walling, weatherboard,
   wood