The Collaborative International Dictionary of English v.0.48:

Overweight \O"ver*weight"\, a.
   1. Overweighing; excessive. [Obs.] "Of no overweight worth."
      --Fuller.
      [1913 Webster]

   2. Having a weight in excess of what is normal, proper, or
      expected; as, the doctor recommends dieting to all his
      overweight patients; overweight luggage will incur an
      extra charge.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Overweight \O"ver*weight`\, n.
   1. Weight over and above what is required by law or custom.
      [1913 Webster]

   2. Superabundance of weight; preponderance.
      [1913 Webster]

WordNet (r) 3.0 (2006):

overweight
    adj 1: usually describes a large person who is fat but has a
           large frame to carry it [syn: fleshy, heavy,
           overweight]
    n 1: the property of excessive fatness [syn: corpulence,
         overweight, stoutness, adiposis]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "overweight":
   adipose, avoirdupois, bear hard upon, beef, beefiness, beefy,
   big-bellied, bloated, blowzy, bosomy, brawny, burden, burly,
   burthen, buxom, charge, choke, chubby, chunky, congest, congestion,
   corpulent, cram, crowd, cumber, deadweight, distended, drench,
   dumpy, encumber, engorgement, fat, fatness, fattish, fleshy,
   freight, full, giantism, gigantism, glut, gluttonize, gorge,
   gravity, gross, gross weight, hamper, handicap, heaviness, heavy,
   heavyset, heavyweight, heft, heftiness, hefty, hippy, hyperemia,
   hypertrophy, imposing, jam, jam-pack, lade, leaden, liveweight,
   load, lusty, massive, massy, meaty, neat weight, net, net weight,
   obese, oppress, outbalance, outpoise, outsize, outsized, outweigh,
   overbalance, overbig, overbrimming, overburden, overcharge,
   overdeveloped, overdevelopment, overfed, overfeed, overfill,
   overflow, overfreight, overfullness, overgrown, overgrowth,
   overlade, overlarge, overlargeness, overload, overpoise, oversized,
   overspill, overstuff, overtax, overweigh, pack, paunchy, plethora,
   plump, podgy, ponderability, ponderosity, ponderous, ponderousness,
   portly, potbellied, poundage, press hard upon, pudgy, puffy, pursy,
   repletion, rest hard upon, roly-poly, rotund, saddle, satiate,
   satiety, saturate, saturation, soak, square, squat, squatty,
   stalwart, stocky, stout, strapping, stuff, supercharge,
   supersaturate, supersaturation, surcharge, surfeit, swollen, tax,
   thick-bodied, thickset, tonnage, too big, top-heavy, tubby,
   underweight, weigh heavy on, weigh on, weigh upon, weight,
   weightiness, weighty, well-fed