The Collaborative International Dictionary of English v.0.48:

Frank \Frank\, n. [See Frank, a.]
   The privilege of sending letters or other mail matter, free
   of postage, or without charge; also, the sign, mark, or
   signature denoting that a letter or other mail matter is to
   go free of postage. Called also the franking privilege.
   [1913 Webster +PJC]

         I have said so much, that, if I had not a frank, I must
         burn my letter and begin again.          --Cowper.
   [1913 Webster]