The Collaborative International Dictionary of English v.0.48:

Billowy \Bil"low*y\, a.
   Of or pertaining to billows; swelling or swollen into large
   waves; full of billows or surges; resembling billows.
   [1913 Webster]

         And whitening down the many-tinctured stream,
         Descends the billowy foam.               --Thomson.
   [1913 Webster] Billposter

WordNet (r) 3.0 (2006):

billowy
    adj 1: characterized by great swelling waves or surges; "billowy
           storm clouds"; "the restless billowing sea"; "surging
           waves" [syn: billowy, billowing(a), surging]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "billowy":
   bagging, baggy, ballooning, bellying, bent, billowing, bloated,
   bosomy, bulbose, bulbous, bulging, bumped, bumpy, bunched, bunchy,
   curvaceous, curvate, curvated, curve, curved, curvesome, curviform,
   curvilineal, curvilinear, curving, curvy, distended, geosynclinal,
   hillocky, hummocky, incurvate, incurvated, incurved, incurving,
   labyrinthine, mazy, meandering, moutonnee, pneumatic, potbellied,
   pouching, recurvate, recurvated, recurved, recurving, rolling,
   rounded, serpentine, sinuous, surgy, swelling, tortuous, undulant,
   undulate, undulated, undulating, undulative, undulatory,
   verrucated, verrucose, warty, wavy