The Collaborative International Dictionary of English v.0.48:

Underclothes \Un"der*clothes`\, n. pl.
   Clothes worn under others, especially those worn next the
   skin for warmth; underwear.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underclothes
    n 1: undergarment worn next to the skin and under the outer
         garments [syn: underwear, underclothes,
         underclothing]