Sir is a polite, respectful way to address a man, especially one you don't know well or who holds a position of authority. Students often call their male teachers “sir,” and you might say “Excuse me, sir” to a stranger you're asking for help. The word shows respect without being overly formal or stiff.

In the military, soldiers address their officers as “sir” as a sign of respect for their rank. In customer service, employees often greet male customers as “sir” to be courteous and professional. Using “sir” doesn't mean you think someone is better than you; it simply shows good manners and consideration.

“Sir” is also a formal title given to men who have been knighted by the British monarch for exceptional achievements. Sir Isaac Newton earned his title for his groundbreaking work in science, and Sir Edmund Hillary received his for being one of the first people to reach the summit of Mount Everest. When “Sir” appears before someone's first name, it indicates they've been honored this way.

A common female equivalent is “ma'am” or, for a knighted woman, “Dame.”