The Collaborative International Dictionary of English v.0.48:

Under-garment \Un"der-gar`ment\, n.
   A garment worn below another.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undergarment
    n 1: a garment worn under other garments [syn: undergarment,
         unmentionable] [ant: outer garment, overgarment]