Meaning

• A man of a higher rank or position.

• A respectful term of address to a man of higher rank or position, particularly:

• A respectful term of address to an adult male (often older), especially if his name or proper title is unknown.

  "Excuse me, sir, do you know the way to the art museum?"

Meaning

• To address (someone) using "sir".

  "Sir, yes, sir! Don't you sir me, private! I work for a living!"