Sir is an honorific used as a title (see Knight) and in several other modern contexts. It was once used (without the person's name) as a courtesy title among equals, but in common usage it's now usually reserved for one of superior rank or status, such as an educator or commanding officer, or in age (especially by a minor); as a form of address from a merchant to a customer; in formal correspondence (Dear Sir, Right Reverend Sir); or to a stranger (Sir, you've dropped your hat). The equivalent… (More on Sir)