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English definition of “sir”

sir

noun [as form of address] uk   /sɜːr/ us    /sɝː/ (also Sir) formal
B1 used as a formal and polite way of speaking to a man, especially one who you are providing a service to or who is in a position of authority: Would you like to see the menu, sir? "Did you hear what I said?" "Yes, Sir."
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mainly UK sometimes used by children to address or refer to teachers who are men: Can I go to the toilet, sir? Indian English used after the name of a man, especially one who is in a position of authority, in order to be polite or show respect: John Sir Dear Sir B2 used to begin a formal letter to a man whose name you do not know. "Dear Sirs" is an old fashioned way of beginning a letter to a company. Dear Sir or Madam used to begin a formal letter when you do not know if the person you are writing to is a man or a woman
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(Definition of sir noun from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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