Longman Dictionary of Contemporary English - sir

~ n 1 spoken a way of addressing a man, for example a male customer in a shop or a military officer that shows respect  ("Report back to me in an hour, sergeant." "Yes, sir." | Can I help you, sir?)  (- compare madam (1) BrE ma'am AmE) 2 AmE spoken used to get the attention of a man whose name you do not know  (Sir! You dropped your wallet!)  (- compare ma'am) 3 Dear Sir used at the beginning of a formal letter to a man 4 Sir a title used before the first name of a knight or baronet  (Sir James Wilson | Sir Jasper) 5 BrE spoken used by children at school as a way of addressing or talking about a male teacher  (Sir, I've forgotten my homework. | Look out - sir's coming back!)  (- compare miss2 (2)) 6 no sir! also no siree! AmE old-fashioned spoken used to emphasize that you do not want something, will not accept something etc  (I will not have that man in my home, no sir!)