The Collaborative International Dictionary of English v.0.48:

mac \mac\ n.
   Shortened form of mackintosh, a waterproof raincoat made of
   rubberized fabric.

   Syn: mackintosh, mac, mack.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Mackintosh \Mack"in*tosh\, n.
   A waterproof outer garment; -- so called from the name of the
   inventor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mackintosh
    n 1: a lightweight waterproof (usually rubberized) fabric [syn:
         mackintosh, macintosh]
    2: a waterproof raincoat made of rubberized fabric [syn:
       macintosh, mackintosh, mac, mack]