Draws on a line from 《古诗十九首》 (Nineteen Old Poems, Han): 著以长相思，缘以结不解 - 'filled with long longing, the border sewn with a knot that cannot be untied.' The image of the unloosenable knot survives in 不解之缘.
usage
Almost always follows 结下/结有/有 + (object) + 不解之缘. The 'object' is usually a place, craft, art form, or field of study - something you've committed to over the long run.
