The Collaborative International Dictionary of English v.0.48:

Mackinaw blanket \Mack"i*naw blan"ket\, Mackinaw \Mack"i*naw\
   [From Mackinac, the State of Michigan, where blankets and
   other stores were distributed to the Indians.]
   1. A thick blanket formerly in common use in the western part
      of the United States. [WordNet sense 2]
      [1913 Webster]

   2. a heavy woolen cloth heavily napped and felted, often with
      a plaid design. [WordNet sense 4]
      [WordNet 1.5] Mackinaw

WordNet (r) 3.0 (2006):

Mackinaw blanket
    n 1: a thick plaid blanket formerly used in the northwestern
         United States [syn: mackinaw, Mackinaw blanket]