sir -
The titular prefix given to a knight or baronet.
Wikipedia
Sir is an honorific used as a title, or as a courtesy title to address a man without using his given or family name in some English speaking cultures. It is often used in formal correspondence (Dear Sir, Right Reverend Sir). The term is often reserved for use only towards equals, one of superior rank or status, such as an educator or commanding officer, an elder (especially by a minor), or as a form of address from a merchant to a customer.
