Source : Webster's Revised Unabridged Dictionary (1913)

Mackinaw coat \Mackinaw coat\
   A short, heavy, double-breasted plaid coat, the design of
   which is large and striking. [Local, U. S.]

Source : WordNet®

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