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: Webster's Revised Unabridged Dictionary (1913)