The story
Supposedly used as a theft-detection system; thieves would retie bags with a more common reef knot, revealing the theft.
Etymology from Wiktionary, CC BY-SA 4.0.
thief knot
Today it means: A knot for tying together two ends of rope; similar to a reef knot, but with the tails (short ends) on opposite sides; weaker than a reef knot. Full definition →