The Collaborative International Dictionary of English v.0.48:

Petticoat \Pet"ti*coat\, n. (Zool.) [Petty + coat.]
   A loose under-garment worn by women, and covering the body
   below the waist.
   [1913 Webster]

   Petticoat government, government by women, whether in
      politics or domestic affairs. [Colloq.]

   Petticoat pipe (Locomotives), a short, flaring pipe
      surrounding the blast nozzle in the smoke box, to equalize
      the draft.
      [1913 Webster]

WordNet (r) 3.0 (2006):

petticoat
    n 1: undergarment worn under a skirt [syn: petticoat, half-
         slip, underskirt]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "petticoat":
   Balmoral, Skivvies, bloomers, body stocking, brassiere, briefs,
   bustle, camisole, chemise, combination, corset, crinoline, distaff,
   drawers, female, feminine, foundation garment, gentlewomanlike,
   girlish, gynecic, gynecoid, gynic, kittenish, ladylike,
   little-girlish, maidenly, matronal, matronlike, matronly,
   muliebral, pannier, panties, pants, shift, shorts, singlet, slip,
   smock, tournure, underpants, underskirt, undervest, union suit,
   vest, womanish, womanlike, womanly