The Collaborative International Dictionary of English v.0.48:

Mackinaw \Mack"i*naw\, Mackinaw coat \Mack"i*naw coat\
   A short, heavy, double-breasted plaid coat, the design of
   which is large and striking. [Local, U. S.]
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Mackinaw coat
    n 1: a short plaid coat made of made of thick woolen material
         [syn: mackinaw, Mackinaw coat]