The Collaborative International Dictionary of English v.0.48:

Lissom \Lis"som\, Lissome \Lis"some\ (l[i^]s"s[u^]m), a. [For
   lithesome.]
   1. Limber; supple; flexible; lithe; lithesome.
      [1913 Webster]

            Straight, but as lissome as a hazel wand.
                                                  --Tennyson.
      [1913 Webster]

   2. Light; nimble; active. --Halliwell. -- Lis"some*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lissomeness
    n 1: the gracefulness of a person or animal that is flexible and
         supple [syn: lissomeness, litheness, suppleness]