Meaning of “sir” - Learner’s Dictionary

sir

noun uk us /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?
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...
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 Richard

(Definition of “sir” from the Cambridge Learner’s Dictionary © Cambridge University Press)