The Collaborative International Dictionary of English v.0.48:

dressed to kill \dressed to kill\ pred. adj.
   attired in strikingly attractive clothing; -- usually of
   women.
   [PJC]

WordNet (r) 3.0 (2006):

dressed to kill
    adj 1: dressed in fancy or formal clothing [syn: dressed(p),
           dressed-up, dressed to the nines(p), dressed to
           kill(p), dolled up, spruced up, spiffed up,
           togged up]

Moby Thesaurus II by Grady Ward, 1.0:

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