Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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