The Collaborative International Dictionary of English v.0.48:

Placket \Plack"et\, n. [F. plaquer to lay or clap on. See
   Placard.]
   1. A petticoat, esp. an under petticoat; hence, a cant term
      for a woman. [Obs.] --Beau. & Fl.
      [1913 Webster]

   2. The opening or slit left in a petticoat or skirt for
      convenience in putting it on; -- called also placket
      hole.
      [1913 Webster]

   3. A woman's pocket.
      [1913 Webster]

WordNet (r) 3.0 (2006):

placket
    n 1: a piece of cloth sewn under an opening

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "placket":
   air hole, armhole, blowhole, bullet-hole, bunghole, cringle,
   deadeye, eye, eyelet, gasket, grommet, guide, keyhole, knothole,
   loop, loophole, manhole, mousehole, peephole, pigeonhole, pinhole,
   placket hole, porthole, punch-hole, spiracle, tap, vent,
   venthole