The Collaborative International Dictionary of English v.0.48:

Undershirt \Un"der*shirt`\, n.
   A shirt worn next the skin, under another shirt; -- called
   also undervest.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undershirt
    n 1: a collarless men's undergarment for the upper part of the
         body [syn: singlet, vest, undershirt]