The Collaborative International Dictionary of English v.0.48:

Winsome \Win"some\, a. [Compar. Winsomer; superl.
   Winsomest.] [AS. wynsum, fr. wynn joy; akin to OS. wunnia,
   OHG. wunna, wunni, G. wonne, Goth. wunan to rejoice (in
   unwunands sad), AS. wunian to dwell. ????. See Win, v. t.,
   Wont, a.]
   [1913 Webster]
   1. Cheerful; merry; gay; light-hearted.
      [1913 Webster]

            Misled by ill example, and a winsome nature.
                                                  --Jeffrey.
      [1913 Webster]

   2. Causing joy or pleasure; gladsome; pleasant.
      [1913 Webster]

            Still plotting how their hungry ear
            That winsome voice again might hear.  --Emerson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

winsome
    adj 1: charming in a childlike or naive way

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "winsome":
   admirable, adorable, alluring, angelic, appealing, appetizing,
   attractive, beaming, beguiling, bewitching, blandishing, blithe,
   blithesome, bright, bright and sunny, cajoling, captivating,
   caressable, catching, charismatic, charming, cheerful, cheery,
   coaxing, come-hither, coquettish, cuddlesome, delightful, dulcet,
   elated, enchanting, engaging, enravishing, enthralling, enticing,
   entrancing, eupeptic, euphoric, exalted, exciting, exhilarated,
   exotic, exquisite, fascinating, fetching, flirtatious, flushed,
   genial, glad, gladsome, glamorous, glowing, happy, heart-robbing,
   high, hopeful, hypnotic, in good spirits, in high spirits,
   interesting, intriguing, inviting, irrepressible, irresistible,
   kissable, laughing, likable, lovable, lovely, lovesome, luxurious,
   mesmeric, mouth-watering, of good cheer, optimistic, piquant,
   pleasant, prepossessing, provocative, provoquant, radiant,
   ravishing, riant, rosy, sanguine, sanguineous, seducing, seductive,
   sensuous, seraphic, siren, sirenic, smiling, sparkling,
   spellbinding, spellful, sunny, sweet, taking, tantalizing, teasing,
   tempting, thrilling, tickling, titillating, titillative,
   voluptuous, winning, witching