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English definition of “sir”

sir

noun
 
 
/sɜːr/
POLITE NAME (also Sir) B1 You call a man 'sir' when you are speaking to him politely: Excuse me, sir, is this seat taken?Names and titles
FORMAL LETTER B2 You write 'Sir' at the beginning of a formal letter to a man when you do not know his name: Dear Sir, I am writing to...Names and titlesWritten greetingsLetters, notes and cards
Sir a title used in the UK before the name of a man who has been officially respected or who has a high social rank: Sir Cliff RichardRoyalty, aristocracy and titles
(Definition of sir from the Cambridge Learners Dictionary © Cambridge University Press)
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