the.com/love knot

means An ornamental knot or looped design, tied with no visible beginning or end, used as a symbol or token of romantic devotion.

from Love knots turn up in European decorative art and verse from the medieval period onward — the endless interlacing loop was an easy, visual metaphor for a bond that never terminates. The design itself is old and widespread, related to Celtic knotwork and other interlace traditions that long predate any romantic meaning; lovers and poets simply adopted a shape that already meant 'unbroken' and gave it a Valentine's job. By the Renaissance, actual knotted ribbons and jewelry called 'love knots' were exchanged as tokens, and the phrase settled into English as shorthand for any emblem of entwined, supposedly endless affection.