noun
plural sirs
[noncount] a - used without a name as a form of polite address to a man you do not know
b - used without a name as a form of polite address to a man of rank or authority (such as a military or police officer, teacher, or master)
Sir [count] - used without a name as a form of address at the beginning of a formal letter
Sir [noncount] - used as a title before the name of a knight or baronet
[noncount] US informal - used in the phrases no sir and yes sir for emphasis, to show surprise, etc.