sir S1 W3 /sə; strong sɜː $ sər strong sɜːr/ noun
[Date: 1200-1300; Origin: ⇒ sire¹]

1. spoken used when speaking to a man in order to be polite or show respect:
‘Report back to me in an hour, sergeant.’ ‘Yes, sir.’
Can I help you, sir?
Sir! You dropped your wallet. ⇒ madam(1), ma'am

2. Dear Sir/Sirs used at the beginning of a formal letter to a man or to people you do not know

3. Sir a title used before the first name of a knight or baronet:
Sir Paul McCartney
Sir Jasper

4. British English spoken used by children at school when speaking to or talking about a male teacher:
Sir, I’ve forgotten my homework.
Look out – sir’s coming back! ⇒ miss²(4)

5. no/yes sir! (also no/yes siree! especially American English) old-fashioned spoken used to emphasize a statement or an answer to a question:
I will not have that man in my home, no sir!

[TahlilGaran] Dictionary of Contemporary English ▲