The Collaborative International Dictionary of English v.0.48:

frank \frank\ (fr[a^][ng]k), v. t. [imp. & p. p. franked
   (fr[a^][ng]kt); p. pr. & vb. n. franking.]
   1. To send by public conveyance free of expense. --Dickens.
      [1913 Webster]

   2. To extempt from charge for postage, as a letter, package,
      or packet, etc.
      [1913 Webster]