The slip knot was a knot designed to snare wild game. By association, could it also mean the snares of love? — the knots called “lacs des amoureux” (lovers' slip knots) in France were traditionally used by sailors in their marriage proposal: they sent their chosen one the design shown in this sketch, BUT WITHOUT PULLING THE KNOT TIGHT. If the girl sent the knot back tied, it symbolized the two lovers' hearts entwined, just like in the photograph. In other words, wedding preparations should get underway immediately. If the knot came back, alas, untied... I think you've got the picture.