Source : Webster's Revised Unabridged Dictionary (1913)

Underclothes \Un"der*clothes`\, n. pl.
   Clothes worn under others, especially those worn next the
   skin for warmth.

Source : WordNet®

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