The Collaborative International Dictionary of English v.0.48:

Willowy \Wil"low*y\, a.
   1. Abounding with willows.
      [1913 Webster]

            Where willowy Camus lingers with delight. --Gray.
      [1913 Webster]

   2. Resembling a willow; pliant; flexible; pendent; drooping;
      graceful.
      [1913 Webster]

WordNet (r) 3.0 (2006):

willowy
    adj 1: slender and graceful [syn: gracile, willowy]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "willowy":
   adaptable, airy, attenuate, attenuated, bendable, bending, boyish,
   compliant, delicate, diaphanous, diluted, ductile, elastic,
   ethereal, extensible, extensile, fabricable, facile, fictile, fine,
   fine-drawn, finespun, flexible, flexile, flexuous, flimsy,
   formable, formative, frail, gauzy, girlish, giving, gossamer,
   gracile, impressible, impressionable, insubstantial, lacy, light,
   like putty, limber, lissome, lithe, lithesome, malleable, misty,
   moldable, papery, plastic, pliable, pliant, rare, rarefied,
   receptive, responsive, sensitive, sequacious, shapable, slender,
   slenderish, slight, slight-made, slim, slimmish, slinky, small,
   springy, submissive, subtle, supple, susceptible, svelte,
   sylphlike, tenuous, thin, thin-bodied, thin-set, thin-spun,
   thinnish, threadlike, tractable, tractile, vague, wasp-waisted,
   watered, watered-down, watery, weak, whippy, wiredrawn, wispy,
   yielding