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]