The Collaborative International Dictionary of English v.0.48:

Dapper \Dap"per\, a. [OE. daper; prob. fr. D. dapper brave,
   valiant; akin to G. tapfer brave, OHG. taphar heavy, weighty,
   OSlav. dobr[u^] good, Russ. dobrui. Cf. Deft.]
   Little and active; spruce; trim; smart; neat in dress or
   appearance; lively.
   [1913 Webster]

         He wondered how so many provinces could be held in
         subjection by such a dapper little man.  --Milton.
   [1913 Webster]

         The dapper ditties that I wont devise.   --Spenser.
   [1913 Webster]

         Sharp-nosed, dapper steam yachts.        --Julian
                                                  Hawthorne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dapper
    adj 1: marked by up-to-dateness in dress and manners; "a dapper
           young man"; "a jaunty red hat" [syn: dapper, dashing,
           jaunty, natty, raffish, rakish, spiffy,
           snappy, spruce]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "dapper":
   bandbox, braw, chic, chichi, classy, clothes-conscious,
   cosmopolitan, dashing, doggy, dressed to advantage,
   dressed to kill, dressy, elegant, exquisite, fashionable, genteel,
   jaunty, natty, neat, nifty, nobby, posh, rakish, recherche, ritzy,
   sassy, sharp, showy, sleek, smart, smug, snazzy, soigne, soignee,
   sophisticated, spiffy, spruce, style-conscious, stylish, swank,
   swanky, swell, tricksy, trig, trim, well-dressed, well-groomed