The Collaborative International Dictionary of English v.0.48:

Dandy \Dan"dy\ (d[a^]n"d[y^]), n.; pl. Dandies
   (d[a^]n"d[i^]z). [Cf. F. dandin, ninny, silly fellow,
   dandiner to waddle, to play the fool; prob. allied to E.
   dandle. Senses 2 & 3 are of uncertain etymology.]
   1. One who affects special finery or gives undue attention to
      dress; a fop; a coxcomb.
      [1913 Webster]

   2. (Naut.)
      (a) A sloop or cutter with a jigger on which a lugsail is
          set.
      (b) A small sail carried at or near the stern of small
          boats; -- called also jigger, and mizzen.
          [1913 Webster]

   3. A dandy roller. See below.
      [1913 Webster]

   Dandy brush, a yard whalebone brush.

   Dandy fever. See Dengue.

   Dandy line, a kind of fishing line to which are attached
      several crosspieces of whalebone which carry a hook at
      each end.

   Dandy roller, a roller sieve used in machines for making
      paper, to press out water from the pulp, and set the
      paper. Dandy-cock

WordNet (r) 3.0 (2006):

dandy
    adj 1: very good; "he did a bully job"; "a neat sports car";
           "had a great time at the party"; "you look simply
           smashing" [syn: bang-up, bully, corking,
           cracking, dandy, great, groovy, keen, neat,
           nifty, not bad(p), peachy, slap-up, swell,
           smashing]
    n 1: a man who is much concerned with his dress and appearance
         [syn: dandy, dude, fop, gallant, sheik, beau,
         swell, fashion plate, clotheshorse]
    2: a sailing vessel with two masts; a small mizzen is aft of the
       rudderpost [syn: yawl, dandy]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "dandy":
   Beau Brummel, OK, ace, ace-high, arbiter of fashion, bad, bang-up,
   beau, beaut, blade, blood, bonzer, boss, boulevardier, buck, bully,
   but good, capital, clotheshorse, clubman, clubwoman, conceited,
   cool, corker, corking, coxcomb, coxcombical, crackerjack, daisy,
   dandified, darb, deb, debutante, delicious, dilly, dream, ducky,
   dude, exquisite, fab, famous, fashion plate, fashionable, fine,
   fine and dandy, fine gentleman, first-class, first-rate,
   first-string, fop, foppish, fribble, gallant, gear, glorious,
   grand, great, groovy, heavy, honey, hot, humdinger, hunky-dory,
   jack-a-dandy, jackanapes, jam-up, just dandy, keen, killer-diller,
   knockout, lady-killer, lollapaloosa, lounge lizard, lulu, macaroni,
   man-about-town, marvy, masher, mean, mondain, mondaine, neat,
   nifty, nobby, okay, out of sight, peach, peachy, peachy-keen, pip,
   pippin, popinjay, prime, puppy, rake, ripping, rum, scrumptious,
   slap-up, smashing, socialite, solid, something else, spark,
   spectacular, spiffing, spiffy, splendid, sport, stunning, subdeb,
   subdebutante, superior, sweetheart, swell, swinger, taste-maker,
   terrific, the nuts, tone-setter, tough, trend-setter, whiz,
   wizard