[1] : 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
see also:
frank franking privilege 

Privilege >> P Words
Privilege, definition of term: Privilege
privilege_pag1.html

Franking >> F Words
Franking, definition of term: Franking
franking_pag1.html