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]