The Collaborative International Dictionary of English v.0.48:

Underclothing \Un"der*cloth`ing\, n.
   Same as Underclothes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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