The Collaborative International Dictionary of English v.0.48:

Underskirt \Un"der*skirt`\, n.
   A petticoat; the foundation skirt of a draped dress.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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