The Collaborative International Dictionary of English v.0.48:

Lightsome \Light"some\ (l[imac]t"s[u^]m), a.
   1. Having light; lighted; not dark or gloomy; bright.
      [1913 Webster]

            White walls make rooms more lightsome than black.
                                                  --Bacon.
      [1913 Webster]

   2. Gay; airy; cheering; exhilarating.
      [1913 Webster]

            That lightsome affection of joy.      --Hooker.
      -- Light"some*ly, adv. -- Light"some*ness, n.
      [1913 Webster]

            Happiness may walk soberly in dark attire, as well
            as dance lightsomely in a gala dress. --Hawthorne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lightsomeness
    n 1: the cheerful feeling you have when nothing is troubling you
         [syn: carefreeness, insouciance, lightheartedness,
         lightsomeness]
    2: the gracefulness of a person or animal that is quick and
       nimble [syn: agility, legerity, lightness,
       lightsomeness, nimbleness]
    3: the trait of being lighthearted and frivolous [syn:
       lightsomeness, lightness]