Definition of vulgar tongue in English:

vulgar tongue

noun

dated the vulgar tongue
  • The national or vernacular language of a people (used typically to contrast such a language with Latin).

    ‘Happy birthday to one of Ireland's greatest comic writers - Brian Ó Nualláin, or Brian Nolan in the vulgar tongue.’
    ‘And as we've heard, he was opposed not just on political grounds, but also on the grounds that a vulgar tongue like the modern English of the day, couldn't possibly convey the Word of God with the right sort of beauty or dignity.’