The Collaborative International Dictionary of English v.0.48:

Sheathing \Sheath"ing\, p. pr. & a. from Sheathe.
   Inclosing with a sheath; as, the sheathing leaves of grasses;
   the sheathing stipules of many polygonaceous plants.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sheathing \Sheath"ing\, n.
   That which sheathes. Specifically:
   (a) The casing or covering of a ship's bottom and sides; the
       materials for such covering; as, copper sheathing.
   (b) (Arch.) The first covering of boards on the outside wall
       of a frame house or on a timber roof; also, the material
       used for covering; ceiling boards in general.
       [1913 Webster]

WordNet (r) 3.0 (2006):

sheathing
    n 1: protective covering consisting, for example, of a layer of
         boards applied to the studs and joists of a building to
         strengthen it and serve as a foundation for a weatherproof
         exterior [syn: sheathing, overlay, overlayer]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "sheathing":
   beam, billet, blanketing, blocking, blotting out, board, boarding,
   case, casing, cementwork, clapboard, cloaking, clouding, coating,
   cord, cordwood, coverage, covering, curtaining, deal, driftwood,
   eclipse, eclipsing, encasement, enveloping, envelopment,
   enwrapment, enwrapping, firewood, hardwood, hiding, incrustation,
   lath, lathing, lathwork, laying on, log, lumber, mantling, masking,
   obduction, obscuring, occultation, overlaying, overspreading,
   panelboard, paneling, panelwork, pargeting, plank, planking,
   plasterwork, plyboard, plywood, pole, post, puncheon, screening,
   shake, sheath, sheathing board, sheeting, shielding, shingle,
   shrouding, sideboard, siding, slab, slat, softwood, splat, stave,
   stick, stick of wood, stovewood, stuccowork, superimposition,
   superposition, three-by-four, timber, timbering, timberwork,
   two-by-four, upholstering, upholstery, veiling, weatherboard, wood,
   wrapping