the.com/glass slipper

means A magical shoe from the Cinderella tale that fits only its rightful owner, now used to describe any impossibly perfect fit or standard — romantic or otherwise.

from From Charles Perrault's 1697 French fairy tale 'Cendrillon,' where the heroine's slipper was 'verre' (glass). A popular but almost certainly false folk theory claims Perrault actually wrote 'vair' (a type of squirrel fur) and that it was mistranslated or mistranscribed into 'verre' — the two words are homophones in French. Most scholars reject this, since fur slippers make little narrative sense and no earlier manuscript supports the switch; Perrault likely intended glass all along, prizing its fairy-tale impossibility and sparkle over practicality.