folk etymology - definition and synonyms

noun [countable] linguistics 

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/ˌfəʊk etɪˈmɒlədʒi/
singularfolk etymology
pluralfolk etymologies
  1. an explanation for the origin of a word or phrase that is believed by many people to be true but that cannot be proved, for example, the belief that ‘poshstands for ‘port out, starboard home