siryoudaoicibaDictYouDict[sir 词源字典]
sir: [13] In common with many other European terms of address for men (such as monsieur and señor), sir goes back ultimately to Latin senior ‘older’ (source also of English senior). This was reduced in Vulgar Latin to *seior, which found its way into Old French as *sieire, later sire. English borrowed this as sire [13], which in weakly-stressed positions (prefixed to names, for instance) became sir.

Other titles based on senior that have found their way into English include French monsieur [15] (literally ‘my sire’), together with its plural messieurs [17], abbreviated to messrs [18]; French seigneur [16]; Spanish señor [17]; and Italian signor [16]. Surly [16] is an alteration of an earlier sirly ‘lordly’, a derivative of sir.

The meaning ‘grumpy’ evolved via an intermediate ‘haughty’.
=> senator, senior, sire, surly[sir etymology, sir origin, 英语词源]
siryoudaoicibaDictYouDict
c. 1300, title of honor of a knight or baronet (until 17c. also a title of priests), variant of sire, originally used only in unstressed position. Generalized as a respectful form of address by mid-14c.; used as a salutation at the beginning of letters from early 15c.