Q: A capitalization question has come up in connection with a novel I’m working on, but I can’t find the answer in The Chicago Manual of Style or anywhere else. Which is correct? “I don’t know, Sir.” Or: “I don’t know, sir.”
A: The word “sir” is lowercase when used in polite address (“May I help you, sir?”).
It’s capitalized in the salutation of a letter (“Dear Sir”) and when part of the given name of a knight or baronet (“Sir Henry had been bludgeoned with the dinner gong”).
This information comes from The American Heritage Dictionary of the English Language (4th ed.).
The word “sir” is a shortened form of “sire,” which we borrowed from Old French in the early 1200s.
Interestingly, “sire” was used for a knight before it came to denote a king or other ruler, according to published references in the Oxford English Dictionary.
And “sir,” of course, hasn’t always been used respectfully. Here’s a contemptuous 14th century example from Chaucer: “Sir olde lecchour, let thi japes be.”