The devil's tongue knot is a fictitious device discussed in the book series "A Series of Unfortunate Events" by Lemony Snicket. In the story, the devil's tongue knot was created by female Finnish pirates in the 15th century. The knot itself does not actually exist.
The devil's tongue knot in the story twists in a most complicated and eerie way. Violet, one of the protagonists of the series, has a number of useful skills, including the making of diverse types of knots. In the book, she uses the devil's tongue knot to tie clothes together so that she can climb a tower.