The Collaborative International Dictionary of English v.0.48:

Latchstring \Latch"string`\, n.
   A string for raising the latch of a door by a person outside.
   It is fastened to the latch and passed through a hole above
   it in the door.
   [1913 Webster]

   To find the latchstring out, to meet with hospitality; to
      be welcome. (Intrusion is prevented by drawing in the
      latchstring.) [Colloq. U.S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

latchstring
    n 1: opener consisting of a string that can be passed through a
         hole in a door for raising the latch from outside