The Collaborative International Dictionary of English v.0.48:

Sweater \Sweat"er\, n.
   1. One who sweats.
      [1913 Webster]

   2. One who, or that which, causes to sweat; as:
      (a) A sudorific.
      (b) A woolen jacket or jersey worn by athletes.
      (c) An employer who oppresses his workmen by paying low
          wages. [Slang]
          [1913 Webster]

WordNet (r) 3.0 (2006):

sweater
    n 1: a crocheted or knitted garment covering the upper part of
         the body [syn: sweater, jumper]
    2: a person who perspires [syn: perspirer, sweater]