The Collaborative International Dictionary of English v.0.48:

Singlet \Sin"glet\, n.
   An unlined or undyed waistcoat; a single garment; -- opposed
   to doublet. [Prov. Eng.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

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